cpuinfo: fix -Wmissing-prototypes warning

src/cpuinfo/SDL_cpuinfo.c

@@ -21,6 +21,8 @@
 
 #include "SDL_internal.h"
 
+#include "SDL_cpuinfo_c.h"
+
 #if defined(SDL_PLATFORM_WIN32) || defined(SDL_PLATFORM_WINRT) || defined(SDL_PLATFORM_GDK)
 #include "../core/windows/SDL_windows.h"
 #endif