glx: drop xmesaP.h include from xf86glx.c

The declarations for the xfree86-specific XMesa functions were moved up to
xmesa.h, requires Mesa as of 2007-04-13.

GL/mesa/X/xf86glx.c

@@ -45,7 +45,6 @@ SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 #include <glxdrawable.h>
 #include <glxcontext.h>
 #include <glxutil.h>
-#include "xmesaP.h"
 
 #include "glcontextmodes.h"
 #include "os.h"