openbsc: Fix build failure

Add explicit reference to osmo-gbproxy.service to fix build issue.

ERROR: QA Issue: openbsc: Files/directories were installed but not shipped
 /lib/systemd/system/osmo-gbproxy.service
ERROR: QA run found fatal errors. Please consider fixing them.
diff --git a/recipes-osmocom/openbsc/openbsc.inc b/recipes-osmocom/openbsc/openbsc.inc
index b8eaca5..a965040 100644
--- a/recipes-osmocom/openbsc/openbsc.inc
+++ b/recipes-osmocom/openbsc/openbsc.inc
@@ -93,6 +93,8 @@
 
 FILES_ipaccess-utils = " ${bindir}/ipaccess-find ${bindir}/ipaccess-config ${bindir}/ipaccess-proxy "
 FILES_osmo-bsc-nat = " ${bindir}/osmo-bsc_nat "
-FILES_osmo-gbproxy = " ${bindir}/osmo-gbproxy "
+FILES_osmo-gbproxy = " ${bindir}/osmo-gbproxy \
+		${systemd_unitdir}/system/osmo-gbproxy.service \
+		"
 FILES_osmo-gbproxy-dbg = " ${bindir}/.debug/osmo-gbproxy "