It appeared in revision 1.3 ("Update from lite2.") It's the only one in the string family that has it, and nothing from it is used.
Index: strsep.c =================================================================== RCS file: /cvs/src/lib/libc/string/strsep.c,v retrieving revision 1.6 diff -u -p -r1.6 strsep.c --- strsep.c 8 Aug 2005 08:05:37 -0000 1.6 +++ strsep.c 25 Jan 2014 06:41:18 -0000 @@ -30,7 +30,6 @@ */ #include <string.h> -#include <stdio.h> /* * Get next token from string *stringp, where tokens are possibly-empty