Fix xorg-server on vmware - was: Fix to make xlibs.xf86videovmware build

This pull request along with adding vmware to the default list of options for services.xserver.videoDrivers, see nixos pull request [1] should fix xserver on vmware.

[1] https://github.com/NixOS/nixos/pull/107


This pull request contains a fix for pkgs.xlibs.xf86videovmware that allow it to build by adding the pixman library header files to CFLAGS.

Additional information:
* nixos - 0.2pre4506_...
* xf86videovmware was not building VMware 4.1.4
* This should also allow for the "vmware" option to be used with services.xserver.videoDrivers in `<nixos-config>`