lvm2: Install devmapper.pc to staging/target pkgconfig directory.

Without this change if some package which have dependency on device mapper
and use pkgconfig to check version always fail to find library because
devmapper.pc file wasn't installed.

Signed-off-by: Marek Belisko <marek.belisko@open-nandra.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>