make: fix the order of Makefile.dep for timex

The order of modules in Makefile.dep matters.

Makefile.dep

@@ -47,17 +47,17 @@ ifneq (,$(filter sixlowpan,$(USEMODULE)))
 	USEMODULE += vtimer
 endif
 
+ifneq (,$(filter uart0,$(USEMODULE)))
+	USEMODULE += lib
+	USEMODULE += posix
+endif
+
 ifneq (,$(filter posix,$(USEMODULE)))
 	USEMODULE += uart0
 	USEMODULE += timex
 	USEMODULE += vtimer
 endif
 
-ifneq (,$(filter uart0,$(USEMODULE)))
-	USEMODULE += lib
-	USEMODULE += posix
-endif
-
 ifneq (,$(filter cbor,$(USEMODULE)))
 	USEMODULE += net_help
 endif