intel: add the missing <strings.h> include

It defines the prototype of ffs that fixes the building error
on Android 6.0 64-bit image.

Signed-off-by: Chih-Wei Huang <cwhuang@linux.org.tw>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>

intel/intel_bufmgr_fake.c

@@ -42,6 +42,7 @@
 #include <string.h>
 #include <assert.h>
 #include <errno.h>
+#include <strings.h>
 #include <xf86drm.h>
 #include <pthread.h>
 #include "intel_bufmgr.h"