meson: Mingw64 gcc doesn't recognize system include_type for sdl2

Windows paths result in command lines like "-isystemC:/msys64/..." that
are not recognized by GCC.  "include_type: 'system'" was only included
in an attempt to fix the -Wundef warnings in SDL 2.0.8, but it was not
effective.  Therefore we can fix this by remove the include_type.

Signed-off-by: Yonggang Luo <luoyonggang@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

meson.build

@@ -240,8 +240,7 @@ endif
 
 sdl = not_found
 if have_system
-  sdl = dependency('sdl2', required: get_option('sdl'), static: enable_static,
-                   include_type: 'system')
+  sdl = dependency('sdl2', required: get_option('sdl'), static: enable_static)
   sdl_image = not_found
 endif
 if sdl.found()