SGSN: Fixing build config

Tests for V.42bis, slhc, llc-xid and sndcp-xid are built and
executed on all build configurations, but are only needed when
the sgsn (gprs) is built. This patch adds conditions check
if the tests mentioned abvove are needed or not.

Change-Id: I6921e6198ea7f99fe5276f91cbc522091853bc4e
diff --git a/openbsc/tests/Makefile.am b/openbsc/tests/Makefile.am
index 0ebd2e2..468edd2 100644
--- a/openbsc/tests/Makefile.am
+++ b/openbsc/tests/Makefile.am
@@ -9,10 +9,6 @@
 	trau \
 	subscr \
 	mm_auth \
-	xid \
-	sndcp_xid \
-	slhc \
-	v42bis \
 	$(NULL)
 
 if BUILD_NAT
@@ -40,6 +36,10 @@
 SUBDIRS += \
 	sgsn \
 	oap \
+	xid \
+	sndcp_xid \
+	slhc \
+	v42bis \
 	$(NULL)
 endif
 endif