xorg-server: rebuild drivers in POST_INSTALL

Without this rebuild, we risk to have X modules
with outdated ABI versions. This is not detected
by `lunar fix` and already broke my X twice.

Don't remove this unless you provide a different fix
for this defect.
Merge pull request #181 from v4hn/xorg-server-prevent-abi-break

xorg-server: rebuild drivers in POST_INSTALL