fix: vcpkg stores SDL.h under SDL2 folder

macOS and Linux seem to not, so in order to get CI working again I
should make use of the preprocessor.

FIXME: I think this makes vcpkg a **hard** requirement which perhaps
isn't ideal if I want ZBA to be as easy to build as possible. I should
try ensuring that building w/out vcpkg works as well

libs/imgui/backends/imgui_impl_sdl.cpp

@@ -67,8 +67,14 @@
 #include "imgui_impl_sdl.h"
 
 // SDL
+#if defined(__WIN32__)
+    #include <SDL2/SDL.h>
+    #include <SDL2/SDL_syswm.h>
+#else
     #include <SDL.h>
     #include <SDL_syswm.h>
+#endif
+
 #if defined(__APPLE__)
 #include <TargetConditionals.h>
 #endif