avoid // in copy headers rule

If LOCAL_COPY_HEADERS_TO is not set, then the copy headers rule
target is set to out/target/product/../obj/include//header.h.
If another rule depends on the header without the //, it will
fail.  Don't insert LOCAL_COPY_HEADERS_TO if it is blank.

Change-Id: Ideac7f8e01288ab5cca98645b307945d6ae9e97c