xorg/generate-expr-from-tarballs: map libgbm properly

Is this even used? I hope not...

Author: K900

pkgs/servers/x11/xorg/generate-expr-from-tarballs.pl

@@ -34,7 +34,7 @@
 $pcMap{"libudev"} = "udev";
 $pcMap{"gl"} = "libGL";
 $pcMap{"GL"} = "libGL";
-$pcMap{"gbm"} = "mesa";
+$pcMap{"gbm"} = "libgbm";
 $pcMap{"hwdata"} = "hwdata";
 $pcMap{"\$PIXMAN"} = "pixman";
 $pcMap{"\$RENDERPROTO"} = "xorgproto";