Fix missing #include in SDL_blendmode.h

Regressed when SDL_BlendMode was changed from an enum to a typedef Uint32.

include/SDL3/SDL_blendmode.h

@@ -28,6 +28,8 @@
 #ifndef SDL_blendmode_h_
 #define SDL_blendmode_h_
 
+#include <SDL3/SDL_stdinc.h>
+
 #include <SDL3/SDL_begin_code.h>
 /* Set up for C function definitions, even when using C++ */
 #ifdef __cplusplus