libpayload: add memchr to libc

libfdt requires memchr. Add missing function to libc.

Change-Id: I872026559d16a352f350147c9d7c4be97456a99f
Signed-off-by: Philipp Hug <philipp@hug.cx>
Reviewed-on: https://review.coreboot.org/c/31354
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>

payloads/libpayload/include/string.h

@@ -39,6 +39,7 @@
 void *memset(void *s, int c, size_t n);
 void *memcpy(void *dst, const void *src, size_t n);
 void *memmove(void *dst, const void *src, size_t n);
+void *memchr(const void *s, int c, size_t n);
 int memcmp(const void *s1, const void *s2, size_t len);
 /** @} */