abis: gsm_data_shared.h includes libosmo-abis header, add abis to cflags

gsm_data_shared.h includes e1_input.h of libosmo-abis, add the
LIBOSMOABIS_CFLAGS to Makefile.am, remove AM_LDFLAGS at the same
time as we only build .a archives.
diff --git a/openbsc/src/libmsc/Makefile.am b/openbsc/src/libmsc/Makefile.am
index 5e64453..4a1488d 100644
--- a/openbsc/src/libmsc/Makefile.am
+++ b/openbsc/src/libmsc/Makefile.am
@@ -1,6 +1,6 @@
 INCLUDES = $(all_includes) -I$(top_srcdir)/include -I$(top_builddir)
-AM_CFLAGS=-Wall $(LIBOSMOCORE_CFLAGS) $(LIBOSMOVTY_CFLAGS) $(COVERAGE_CFLAGS)
-AM_LDFLAGS = $(LIBOSMOCORE_LIBS) $(LIBOSMOGSM_LIBS) $(COVERAGE_LDFLAGS)
+AM_CFLAGS=-Wall $(LIBOSMOCORE_CFLAGS) $(LIBOSMOVTY_CFLAGS) \
+	$(LIBOSMOABIS_CFLAGS) $(COVERAGE_CFLAGS)
 
 noinst_LIBRARIES = libmsc.a