pkg/oonf_api: Build with -Wno-format-nonliteral

pkg/oonf_api/Makefile

@@ -18,3 +18,6 @@ $(BINDIR)/$(MODULE).a: $(BINDIR)/oonf_*.a
 	mkdir -p $(BINDIR)/$(MODULE); cd $(BINDIR)/$(MODULE); for var in $?; do ar -x $$var; done; ar -r -c -s $(BINDIR)/$(MODULE).a *.o
 
 include $(RIOTBASE)/pkg/pkg.mk
+ifneq (,$(filter -Wformat-nonliteral -Wformat=2, $(CFLAGS)))
+  CFLAGS += -Wno-format-nonliteral
+endif