"redundant redeclaration" warnings on MSYS2 UCRT

[fabiangreffrath]

This commit #13715 leads to "redundant redeclaration" warnings on MSYS2 UCRT systems, which get caught by -WError and lead to our CI failing [1]:

In file included from D:/a/_temp/msys64/usr/local/include/SDL3/SDL.h:35,
                 from D:/a/woof/woof/textscreen/txt_sdl.h:24,
                 from D:/a/woof/woof/textscreen/txt_main.h:24,
                 from D:/a/woof/woof/textscreen/txt_io.h:21,
                 from D:/a/woof/woof/textscreen/txt_checkbox.c:22:
D:/a/_temp/msys64/usr/local/include/SDL3/SDL_assert.h:131:25: error: redundant redeclaration of '__debugbreak' [-Werror=redundant-decls]
  131 |     extern void __cdecl __debugbreak(void);
      |                         ^~~~~~~~~~~~
In file included from D:/a/_temp/msys64/ucrt64/include/corecrt.h:10,
                 from D:/a/_temp/msys64/ucrt64/include/stdlib.h:9,
                 from D:/a/woof/woof/textscreen/txt_checkbox.c:15:
D:/a/_temp/msys64/ucrt64/include/_mingw.h:605:36: note: previous definition of '__debugbreak' with type 'void(void)'
  605 | __MINGW_INTRIN_INLINE void __cdecl __debugbreak(void)
      |                                    ^~~~~~~~~~~~
cc1.exe: all warnings being treated as errors

[1] https://github.com/fabiangreffrath/woof/actions/runs/18581176802/job/52976207158?pr=2422