{src,tests}/Makefile.am: $(COMMON_LA) is not definied

Change-Id: I4958b5d62645be6ed2a200a50a23c567c9a9f3c0
diff --git a/src/Makefile.am b/src/Makefile.am
index fedc96f..dea4a17 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -161,7 +161,7 @@
 	$(LIBOSMOCORE_LIBS) \
 	$(LIBOSMOCTRL_LIBS) \
 	$(LIBOSMOGSM_LIBS) \
-	$(COMMON_LA)
+	$(NULL)
 endif
 
 if ENABLE_LC15BTS_PHY
@@ -210,6 +210,6 @@
 	$(LIBOSMOCORE_LIBS) \
 	$(LIBOSMOCTRL_LIBS) \
 	$(LIBOSMOGSM_LIBS) \
-	$(COMMON_LA)
+	$(NULL)
 
 #MOSTLYCLEANFILES += testSource testDestination