Add the memcmp() function prototype to builtin/string.h

Add memcmp() to builtin/string.h so that portable third_party
code may build without glibc.

BRANCH=none
BUG=chrome-os-partner:43025,chrome-os-partner:47524
TEST=none
Signed-off-by: nagendra modadugu <ngm@google.com>

Change-Id: Id52c9c76fceac94bf1998958b43f42ad5d5298d3
Reviewed-on: https://chromium-review.googlesource.com/339878
Commit-Ready: Nagendra Modadugu <ngm@google.com>
Tested-by: Nagendra Modadugu <ngm@google.com>
Reviewed-by: Vadim Bendebury <vbendeb@chromium.org>
Reviewed-by: Bill Richardson <wfrichar@google.com>

builtin/string.h

@@ -10,6 +10,7 @@
 
 #include <stddef.h>
 
+int memcmp(const void *s1, const void *s2, size_t len);
 void *memcpy(void *dest, const void *src, size_t len);
 void *memset(void *dest, int c, size_t len);