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>

package/lvm2/lvm2.mk

@@ -21,7 +21,7 @@ LVM2_BINS = \
 
 # Make sure that binaries and libraries are installed with write
 # permissions for the owner.
-LVM2_CONF_OPT += --enable-write_install
+LVM2_CONF_OPT += --enable-write_install --enable-pkgconfig
 
 # LVM2 uses autoconf, but not automake, and the build system does not
 # take into account the CC passed at configure time.