Examples: SDL+OpenGL3: fixed build on ES2 target. (#4492)

examples/example_sdl_opengl3/main.cpp

@@ -8,9 +8,10 @@
 #include "imgui_impl_opengl3.h"
 #include <stdio.h>
 #include <SDL.h>
-#include <SDL_opengl.h>
 #if defined(IMGUI_IMPL_OPENGL_ES2)
 #include <SDL_opengles2.h>
+#else
+#include <SDL_opengl.h>
 #endif
 
 // Main code