build: fix unknown pragma warning with mingw (neovim#22533)

This checks MSVC toolchain with _MSC_VER macro before adding
pragma warning directive. It is specific to MSVC and shows
compiler warning with mingw gcc as following:

main.c:187: warning: ignoring '#pragma warning ' [-Wunknown-pragmas]
  187 | # pragma warning(suppress : 4996)

src/nvim/main.c

@@ -184,8 +184,12 @@ void early_init(mparm_T *paramp)
  ovi.dwOSVersionInfoSize = sizeof(ovi);
  // Disable warning about GetVersionExA being deprecated. There doesn't seem to be a convenient
  // replacement that doesn't add a ton of extra code as of writing this.
-# pragma warning(suppress : 4996)
+# ifdef _MSC_VER
+# pragma warning(suppress : 4996)
  GetVersionEx(&ovi);
+# else
+ GetVersionEx(&ovi);
+# endif
  snprintf(windowsVersion, sizeof(windowsVersion), "%d.%d",
  (int)ovi.dwMajorVersion, (int)ovi.dwMinorVersion);
 #endif