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>

nouveau/nouveau.c

@@ -30,6 +30,7 @@
 #include <stdlib.h>
 #include <stdint.h>
 #include <string.h>
+#include <strings.h>
 #include <stdbool.h>
 #include <assert.h>
 #include <errno.h>

tests/modetest/modetest.c

@@ -50,6 +50,7 @@
 #include <inttypes.h>
 #include <unistd.h>
 #include <string.h>
+#include <strings.h>
 #include <errno.h>
 #include <sys/poll.h>
 #include <sys/time.h>