xlib: call _mesa_warning() instead of fprintf()

We use _mesa_warning() everywhere else in this code.  Change requested
by Rick Irons of Mathworks.

CC: <mesa-stable@lists.freedesktop.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/mesa/drivers/x11/fakeglx.c

@@ -2804,7 +2804,8 @@ Fake_glXCreateContextAttribs(Display *dpy, GLXFBConfig config,
          profileFlags = attrib_list[i + 1];
          break;
       default:
-         fprintf(stderr, "Bad attribute in glXCreateContextAttribs()\n");
+         _mesa_warning(NULL, "Unexpected attribute 0x%x in "
+                       "glXCreateContextAttribs()\n", attrib_list[i]);
          return 0;
       }
    }