Add libnvpair to libzfs pkg-config

Functions such as `fnvlist_lookup_nvlist` need libnvpair to be linked.
Default pkg-config file did not contain it.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Harry Mallon <hjmallon@gmail.com>
Closes #8919

lib/libzfs/libzfs.pc.in

@@ -9,4 +9,4 @@ Version: @VERSION@
 URL: http://zfsonlinux.org
 Requires: libzfs_core
 Cflags: -I${includedir}/libzfs -I${includedir}/libspl
-Libs: -L${libdir} -lzfs
+Libs: -L${libdir} -lzfs -lnvpair