Add missing <strings.h> includes

A couple of files use ffs() without explicitly including strings.h.
Some systems will pull in ffs()'s declaration through another header
anyway, but not when compiling against bionic in AOSP master.

Signed-off-by: Greg Hackmann <ghackmann@google.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>