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
diff --git a/vcl/Library_vclplug_gen.mk b/vcl/Library_vclplug_gen.mk
index c250ca8..f281605 100644
--- a/vcl/Library_vclplug_gen.mk
+++ b/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 \