mui: including missing header and fix some warnings

[bradjc]

This includes the u8g2.h header in mui_u8g2.h. The mui_u8g2.h header relies on the u8g2_uint_t type, but without
including the header the compiler errors on unknown type.

I think this has generally been hidden because most users probably include "u8g2.h" before "mui_u8g2.h". But just re-ordering those includes can cause the build to fail.

This also fixes warnings about internal functions by adding static:

mui.c:303:5: warning: no previous prototype for 'mui_find_uif' [-Wmissing-prototypes]
  303 | int mui_find_uif(mui_t *ui, uint8_t id0, uint8_t id1)
      |     ^~~~~~~~~~~~
mui.c:463:8: warning: no previous prototype for 'mui_find_form' [-Wmissing-prototypes]
  463 | fds_t *mui_find_form(mui_t *ui, uint8_t n)
      |        ^~~~~~~~~~~~~
mui.c:490:9: warning: no previous prototype for 'mui_task_draw' [-Wmissing-prototypes]
  490 | uint8_t mui_task_draw(mui_t *ui)
      |         ^~~~~~~~~~~~~
mui.c:497:9: warning: no previous prototype for 'mui_task_form_start' [-Wmissing-prototypes]
  497 | uint8_t mui_task_form_start(mui_t *ui)
      |         ^~~~~~~~~~~~~~~~~~~
mui.c:503:9: warning: no previous prototype for 'mui_task_form_end' [-Wmissing-prototypes]
  503 | uint8_t mui_task_form_end(mui_t *ui)
      |         ^~~~~~~~~~~~~~~~~
mui.c:519:9: warning: no previous prototype for 'mui_task_find_prev_cursor_uif' [-Wmissing-prototypes]
  519 | uint8_t mui_task_find_prev_cursor_uif(mui_t *ui)
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
mui.c:534:9: warning: no previous prototype for 'mui_task_find_first_cursor_uif' [-Wmissing-prototypes]
  534 | uint8_t mui_task_find_first_cursor_uif(mui_t *ui)
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
mui.c:548:9: warning: no previous prototype for 'mui_task_find_last_cursor_uif' [-Wmissing-prototypes]
  548 | uint8_t mui_task_find_last_cursor_uif(mui_t *ui)
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
mui.c:559:9: warning: no previous prototype for 'mui_task_find_next_cursor_uif' [-Wmissing-prototypes]
  559 | uint8_t mui_task_find_next_cursor_uif(mui_t *ui)
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
mui.c:578:9: warning: no previous prototype for 'mui_task_get_current_cursor_focus_position' [-Wmissing-prototypes]
  578 | uint8_t mui_task_get_current_cursor_focus_position(mui_t *ui)
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
mui.c:590:9: warning: no previous prototype for 'mui_task_read_nth_selectable_field' [-Wmissing-prototypes]
  590 | uint8_t mui_task_read_nth_selectable_field(mui_t *ui)
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
mui.c:602:9: warning: no previous prototype for 'mui_task_find_execute_on_select_field' [-Wmissing-prototypes]
  602 | uint8_t mui_task_find_execute_on_select_field(mui_t *ui)
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
mui.c:654:6: warning: no previous prototype for 'mui_next_field' [-Wmissing-prototypes]
  654 | void mui_next_field(mui_t *ui)