cmake: generate git hash using GetRevisionDescription CMake module

This allows the build system (ninja/make/VS) to detect whether the current
checkout git commit has changed. If so, SDL_revision.h will be updated.

test/testver.c

@@ -15,6 +15,7 @@
 */
 #include <SDL3/SDL.h>
 #include <SDL3/SDL_main.h>
+#include <SDL3/SDL_revision.h>
 
 int main(int argc, char *argv[])
 {