SDL_egl: allow creation of versioned OpenGL contexts with EGL_KHR_create_context

If the EGL extension EGL_KHR_create_context is available, we can use it to
set the core/compatability profile and the minimum OpenGL version.

Use this if it is available to get the context requested by the GL attributes.

include/SDL_egl.h

@@ -27,6 +27,7 @@
 #ifndef _MSC_VER
 
 #include <EGL/egl.h>
+#include <EGL/eglext.h>
 
 #else /* _MSC_VER */