gdbus: Append LDLIBS

Allow makefiles to add additional libraries.

Change-Id: Ia09e9de2dd1c49d92512bed87bb5d906b258316f
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/gdbus.mk b/gdbus.mk
index 220a650..ddba489 100644
--- a/gdbus.mk
+++ b/gdbus.mk
@@ -1,5 +1,5 @@
 PACKAGE_DEPS=gio-unix-2.0 glib-2.0
-LDLIBS=-lopenbmc_intf
+LDLIBS+=-lopenbmc_intf
 
 %.o: %_obj.c
 	$(CC) -c $(ALL_CFLAGS) -o $@ $<