Fix makefile in case a translation does not yield any output

In case there's not any output files yet from a translation, the lang
directory is not created and the build fails.

Signed-off-by: Jean-Noël Avila <jn.avila@free.fr>

Makefile

@@ -28,6 +28,7 @@ update-sources:
 define MAKE_TARGET
 
 $(1)_$(2):
+	@mkdir -p $(2)
 	$(QUIET_LANG) $(2) -f ../makefile.locale $(1) lang=$(2)
 
 $(1): $(1)_$(2)