Update GLDEBUGPROC to the latest spec version.

The missing 'const' on the userParam argument causes problems with ubsan
when ANGLE compiles with the latest headers.

BUG=820848

Change-Id: I516141f132ef7e6a61f3543592df8f0f9b806363
diff --git a/include/GL/glext.h b/include/GL/glext.h
index 04ca939..8036db5 100644
--- a/include/GL/glext.h
+++ b/include/GL/glext.h
@@ -6248,7 +6248,7 @@
 #endif
 
 #ifndef GL_KHR_debug
-typedef void (APIENTRY *GLDEBUGPROC)(GLenum source,GLenum type,GLuint id,GLenum severity,GLsizei length,const GLchar *message,GLvoid *userParam);
+typedef void (APIENTRY *GLDEBUGPROC)(GLenum source,GLenum type,GLuint id,GLenum severity,GLsizei length,const GLchar *message,const GLvoid *userParam);
 #endif
 
 #ifndef GL_NV_vdpau_interop