libcap: build with -fPIC

systemd v247 with updated libcap 2.54 fails to build with:
  libcap.a(cap_proc.o): requires unsupported dynamic reloc
  R_ARM_MOVW_ABS_NC; recompile with -fPIC

This change occured between 2.53 and 2.54 of libcap:
- https://sites.google.com/site/fullycapable/release-notes-for-libcap#h.okdn4r9me10

packages/devel/libcap/package.mk

@@ -12,6 +12,7 @@ PKG_URL="https://www.kernel.org/pub/linux/libs/security/linux-privs/libcap2/${PK
 PKG_DEPENDS_HOST="ccache:host"
 PKG_DEPENDS_TARGET="toolchain"
 PKG_LONGDESC="A library for getting and setting POSIX.1e capabilities."
+PKG_BUILD_FLAGS="+pic"
 
 post_unpack() {
   mkdir -p ${PKG_BUILD}/.${HOST_NAME}