utils/Makefile.am: remove duplicate libosmogsm.la

It's already listed in $(LDADD), no need to append it again.

Change-Id: I03f5f4330e828a938dafd93b9fdb576afb2b7f12
diff --git a/utils/Makefile.am b/utils/Makefile.am
index 6e11dcd..d81d0a6 100644
--- a/utils/Makefile.am
+++ b/utils/Makefile.am
@@ -40,7 +40,7 @@
 			$(top_builddir)/src/gb/libosmogb.la \
 			$(top_builddir)/src/vty/libosmovty.la \
 			$(top_builddir)/src/ctrl/libosmoctrl.la \
-			$(top_builddir)/src/gsm/libosmogsm.la
+			$(NULL)
 osmo_ns_dummy_CFLAGS = $(TALLOC_CFLAGS)
 endif
 endif