package/libcap: fix libcap.pc

package/libcap: fix libcap.pc

libcap builds an incorrect libcap.pc because libdir is pulled from the
host os:

ifndef lib
lib=$(shell ldd /usr/bin/ld|egrep "ld-linux|ld.so"|cut -d/ -f2)
endif

Fix this error by passing lib=lib and prefix in
{HOST_LIBCAP,LIBCAP}_BUILD_CMDS

Fixes:
 - https://bugs.buildroot.org/show_bug.cgi?id=13276

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Reviewed-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>