Establish explicit dependency on selinux file_contexts.

Before this change, because the file_contexts is not in the system,
data or cache directory, the dependency is not reliable.
This fixed the gms build error:
out/target/product/gms/root/file_contexts: No such file or directory

Change-Id: Id8d5b3f4557d43a509ccc00b21dd426a414b7c42
diff --git a/core/Makefile b/core/Makefile
index 49a94c8..7ce82a0 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -570,6 +570,9 @@
 endif
 INTERNAL_USERIMAGES_BINARY_PATHS := $(sort $(dir $(INTERNAL_USERIMAGES_DEPS)))
 
+SELINUX_FC := $(TARGET_ROOT_OUT)/file_contexts
+INTERNAL_USERIMAGES_DEPS += $(SELINUX_FC)
+
 # $(1): the path of the output dictionary file
 define generate-userimage-prop-dictionary
 $(if $(INTERNAL_USERIMAGES_EXT_VARIANT),$(hide) echo "fs_type=$(INTERNAL_USERIMAGES_EXT_VARIANT)" >> $(1))
@@ -579,7 +582,7 @@
 $(if $(BOARD_CACHEIMAGE_PARTITION_SIZE),$(hide) echo "cache_size=$(BOARD_CACHEIMAGE_PARTITION_SIZE)" >> $(1))
 $(if $(INTERNAL_USERIMAGES_SPARSE_EXT_FLAG),$(hide) echo "extfs_sparse_flag=$(INTERNAL_USERIMAGES_SPARSE_EXT_FLAG)" >> $(1))
 $(if $(mkyaffs2_extra_flags),$(hide) echo "mkyaffs2_extra_flags=$(mkyaffs2_extra_flags)" >> $(1))
-$(hide) echo "selinux_fc=$(TARGET_ROOT_OUT)/file_contexts" >> $(1)
+$(hide) echo "selinux_fc=$(SELINUX_FC)" >> $(1)
 endef
 
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