include: Fix glDebugMessageCallbackARB's prototype.

Seems to be a bug in Khronos headers, as

http://www.opengl.org/registry/specs/ARB/debug_output.txt states userParam
is not a const pointer.

Signed-off-by: Dave Airlie <airlied@redhat.com>

include/GL/glext.h

@@ -7573,12 +7573,12 @@ typedef GLsync (APIENTRYP PFNGLCREATESYNCFROMCLEVENTARBPROC) (struct _cl_context
 #ifdef GL_GLEXT_PROTOTYPES
 GLAPI void APIENTRY glDebugMessageControlARB (GLenum source, GLenum type, GLenum severity, GLsizei count, const GLuint *ids, GLboolean enabled);
 GLAPI void APIENTRY glDebugMessageInsertARB (GLenum source, GLenum type, GLuint id, GLenum severity, GLsizei length, const GLchar *buf);
-GLAPI void APIENTRY glDebugMessageCallbackARB (GLDEBUGPROCARB callback, const GLvoid *userParam);
+GLAPI void APIENTRY glDebugMessageCallbackARB (GLDEBUGPROCARB callback, GLvoid *userParam);
 GLAPI GLuint APIENTRY glGetDebugMessageLogARB (GLuint count, GLsizei bufsize, GLenum *sources, GLenum *types, GLuint *ids, GLenum *severities, GLsizei *lengths, GLchar *messageLog);
 #endif /* GL_GLEXT_PROTOTYPES */
 typedef void (APIENTRYP PFNGLDEBUGMESSAGECONTROLARBPROC) (GLenum source, GLenum type, GLenum severity, GLsizei count, const GLuint *ids, GLboolean enabled);
 typedef void (APIENTRYP PFNGLDEBUGMESSAGEINSERTARBPROC) (GLenum source, GLenum type, GLuint id, GLenum severity, GLsizei length, const GLchar *buf);
-typedef void (APIENTRYP PFNGLDEBUGMESSAGECALLBACKARBPROC) (GLDEBUGPROCARB callback, const GLvoid *userParam);
+typedef void (APIENTRYP PFNGLDEBUGMESSAGECALLBACKARBPROC) (GLDEBUGPROCARB callback, GLvoid *userParam);
 typedef GLuint (APIENTRYP PFNGLGETDEBUGMESSAGELOGARBPROC) (GLuint count, GLsizei bufsize, GLenum *sources, GLenum *types, GLuint *ids, GLenum *severities, GLsizei *lengths, GLchar *messageLog);
 #endif