RepositoryExternal.mk: missing dependency on python3 package

This was causing all these "libpython3.3m.so: file not recognized: File
truncated" failures.

Change-Id: Ib814ab4fa43d9c8f20cf9b16ad04e1a71057e595

RepositoryExternal.mk

@@ -3170,6 +3170,7 @@ endef
 
 define gb_LinkTarget__use_python
 $(call gb_LinkTarget__use_python_headers,$(1))
+$(call gb_LinkTarget_use_package,$(1),python3)
 
 ifeq ($(OS),WNT)
 $(call gb_LinkTarget_add_libs,$(1),\