st/wgl: consistently use ifndef guards over pragma once

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Acked-by: Vedran Miletić <vedran@miletic.net>
Acked-by: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>
Reviewed-by: Edward O'Callaghan <funfunctor@folklore1984.net>

src/gallium/state_trackers/wgl/stw_ext_context.h

@@ -22,7 +22,8 @@
  * OTHER DEALINGS IN THE SOFTWARE.
  */
 
-#pragma once
+#ifndef STW_EXT_CONTEXT_H
+#define STW_EXT_CONTEXT_H
 
 #include <windows.h>
 #include <GL/gl.h>
@@ -34,3 +35,4 @@ typedef BOOL (WINAPI *wglDeleteContext_t)(HGLRC hglrc);
 extern wglCreateContext_t wglCreateContext_func;
 extern wglDeleteContext_t wglDeleteContext_func;
 
+#endif /* STW_EXT_CONTEXT_H */