project: thulium: Enable Werror compiler option

Enable compiler option to treat warning as errors.

Change-Id: Ia89b095e68a1f3b899a6be306c1aee5c5ffed61d
diff --git a/project/thulium.mk b/project/thulium.mk
index 217cf50..f9cbe6a 100644
--- a/project/thulium.mk
+++ b/project/thulium.mk
@@ -53,3 +53,5 @@
 ifeq ($(ENABLE_USB30_SUPPORT),1)
 DEFINES += USB30_SUPPORT=1
 endif
+
+CFLAGS += -Werror