ctrlsdl: Fix compilation without OpenGL

Since r3947 ctrlssdl.h breaks the build with a compile
error in build environments that don't have GL.h such as
Mac OS.

[xrmx: define screen_texture anyway to be on the safe side]

Based from a patch from Sidney Markowitz in #3288504

desmume/src/ctrlssdl.h

@@ -94,10 +94,11 @@ struct ctrls_event_config {
   int sdl_quit;
   int boost;
   int fake_mic;
-  GLuint *screen_texture;
 #ifdef HAVE_GL_GL_H
+  GLuint *screen_texture;
   void (*resize_cb)(u16 width, u16 height, GLuint *screen_texture);
 #else
+  void *screen_texture;
   void (*resize_cb)(u16 width, u16 height, void *screen_texture);
 #endif
 };