Use unprefixed SDL include paths for SDL2 and SDL1

When SDL2 is built from source or provided via CMake targets, the
include path points directly to the headers directory, so <SDL.h>
works but <SDL2/SDL.h> does not.  The <SDL2/...> form only works
with system pkg-config installs that add the parent directory.

Use the unprefixed form for SDL2 and SDL1 which is compatible with
both CMake-target and system-install include path layouts.

SDL3 retains the <SDL3/...> prefix as that is the canonical form
established by the SDL3 project.

Author: AJenbo

include/mpqfs/mpqfs.h

@@ -61,11 +61,11 @@
 #   include <SDL3/SDL_iostream.h>
 #   include <SDL3/SDL.h>
 #elif defined(MPQFS_USE_SDL2) && MPQFS_USE_SDL2
-#   include <SDL2/SDL_rwops.h>
-#   include <SDL2/SDL.h>
+#   include <SDL_rwops.h>
+#   include <SDL.h>
 #elif defined(MPQFS_USE_SDL1) && MPQFS_USE_SDL1
-#   include <SDL/SDL_rwops.h>
-#   include <SDL/SDL.h>
+#   include <SDL_rwops.h>
+#   include <SDL.h>
 #endif
 
 /* -----------------------------------------------------------------------

src/mpq_sdl.c

@@ -32,10 +32,10 @@
 #if MPQFS_SDL_VERSION == 3
 #include <SDL3/SDL.h>
 #elif MPQFS_SDL_VERSION == 2
-#include <SDL2/SDL.h>
+#include <SDL.h>
 #else
-#include <SDL/SDL.h>
-#include <SDL/SDL_rwops.h>
+#include <SDL.h>
+#include <SDL_rwops.h>
 #endif
 
 /* -----------------------------------------------------------------------