Add force link option to install target

This allows 'make install' to be invoked more than once.

Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/libopenbmc_intf/Makefile b/libopenbmc_intf/Makefile
index 54bef57..048d8c4 100644
--- a/libopenbmc_intf/Makefile
+++ b/libopenbmc_intf/Makefile
@@ -17,7 +17,7 @@
 install-lib:
 	@mkdir -p $(DESTDIR)$(libdir)
 	install lib$(LIBOBMC).so.1 $(DESTDIR)$(libdir)
-	ln -s lib$(LIBOBMC).so.1 $(DESTDIR)$(libdir)/lib$(LIBOBMC).so
+	ln -sf lib$(LIBOBMC).so.1 $(DESTDIR)$(libdir)/lib$(LIBOBMC).so
 
 clean-lib:
 	rm -f lib$(LIBOBMC).so.1