mui_u8g2.h: add u8g2.h header

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.

csrc/mui_u8g2.h

@@ -65,6 +65,7 @@
 #ifndef MUI_U8G2_H
 #define MUI_U8G2_H
 
+#include "u8g2.h"
 #include "mui.h"
 
 /*==========================================*/