Add header that defines strerror()

It helps to fix a compilation issue with host libc:

core/base/errors_unix.cpp:25:10: error: use of undeclared identifier 'strerror'
  return strerror(error_code);
         ^

Change-Id: I71d4221cc1bfa6f0cc764b40b6b28159994b8e04

base/errors_unix.cpp

@@ -17,6 +17,7 @@
 #include "android-base/errors.h"
 
 #include <errno.h>
+#include <string.h>
 
 namespace android {
 namespace base {