lib: libc: string: Port strtok and strtok_r implementation from bionic

Port strtok and strtok_r implementation from bionic

Change-Id: I81afd1563d307dd09e99cf7565f4e98cb6558847
diff --git a/include/string.h b/include/string.h
index 593ecf2..1d987c3 100644
--- a/include/string.h
+++ b/include/string.h
@@ -53,6 +53,7 @@
 size_t      strcspn(const char *s, const char *) __PURE;
 char       *strstr(char const *, char const *) __PURE;
 char       *strtok(char *, char const *);
+char       *strtok_r(char *s, const char *delim, char **last);
 int         strcoll(const char *s1, const char *s2) __PURE;
 size_t      strxfrm(char *dest, const char *src, size_t n) __PURE;
 char       *strdup(const char *str) __MALLOC;