vendor/lineage: Depend on existing definition of empty

Defined and marked readonly in build/make here:
https://github.com/LineageOS/android_build/blob/7ddc50a066287b804a754951a597668d627478df/common/core.mk#L46

Change-Id: Ic233f0d41d03b28beb8d96c68692271d959fa188
diff --git a/build/core/utils.mk b/build/core/utils.mk
index 23bd4c9..4b29233 100644
--- a/build/core/utils.mk
+++ b/build/core/utils.mk
@@ -1,6 +1,6 @@
 # vars for use by utils
-empty :=
-space := $(empty) $(empty)
+# the following are already defined in build/make:
+# empty space comma newline pound backslash
 colon := $(empty):$(empty)
 underscore := $(empty)_$(empty)