gl/context/egl: change header guard to be unique

The header guard in gst/gl/egl/gstglcontext_egl.h was the same as
gst/gl/egl/egl.h

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/merge_requests/809>

gst-libs/gst/gl/egl/gstglcontext_egl.h

@@ -18,8 +18,8 @@
  * Boston, MA 02110-1301, USA.
  */
 
-#ifndef __GST_GL_EGL_H__
-#define __GST_GL_EGL_H__
+#ifndef __GST_GL_CONTEXT_EGL_H__
+#define __GST_GL_CONTEXT_EGL_H__
 
 #include <gst/gl/gstglcontext.h>
 #include <gst/gl/egl/gstgldisplay_egl.h>
@@ -91,4 +91,4 @@ gpointer            gst_gl_context_egl_get_proc_address     (GstGLAPI gl_api, co
 
 G_END_DECLS
 
-#endif /* __GST_GL_EGL_H__ */
+#endif /* __GST_GL_CONTEXT_EGL_H__ */