Add missing icu_headers dependency

Apparently this was working before by accident since most people have
ICU headers in the system include path on Linux.

Should fix the failing tinderbox.

Change-Id: I796453e66c53b3c64e546c661007a54fa5d866ec

vcl/Library_vclplug_gen.mk

@@ -54,6 +54,7 @@ $(eval $(call gb_Library_use_externals,vclplug_gen,\
 	glew \
 	glm_headers \
 	harfbuzz \
+	icu_headers \
 	icuuc \
 	valgrind \
 	Xrender \