Drop the inconsistent use of #pragma once.

Apparently it's not totally portable, and the #ifdefs work fine (and
equivalently to the pragma, on some compilers).

include/epoxy/egl.h

@@ -48,8 +48,6 @@ extern "C" {
 #define __eglext_h_
 #endif
 
-#pragma once
-
 #include "epoxy/egl_generated.h"
 
 bool epoxy_has_egl_extension(EGLDisplay *dpy, const char *extension);

include/epoxy/glx.h

@@ -51,8 +51,6 @@ extern "C" {
 #define __glxext_h_
 #endif
 
-#pragma once
-
 #include "epoxy/glx_generated.h"
 
 bool epoxy_has_glx_extension(Display *dpy, int screen, const char *extension);

include/epoxy/wgl.h

@@ -46,8 +46,6 @@ extern "C" {
 #define __wglxext_h_
 #endif
 
-#pragma once
-
 #include "epoxy/wgl_generated.h"
 
 bool epoxy_has_wgl_extension(HDC hdc, const char *extension);