stubs missing include for string.h

stubs.cpp gets string.h inherited from private/android_filesystem_config.h
it should not rely on this in the future. The intent is to move fs_config
function into libcutils and thus deprecate any need for string.h in this
include file.

Change-Id: I946ec1979ef5bbb34fbcb4a99bf2cd79280bb2a3