Have the man page catch up with the namespace pollution cleanup that

occurred between 2001-2003.  Thanks to bde for the history lesson[1]
concerning sys/types.h and the many system calls that at one time
(pre-2001) were required by POSIX to include it.

1. http://lists.freebsd.org/pipermail/freebsd-arch/2008-April/008126.html

MFC after:	3 days

lib/libc/sys/mkdir.2

@@ -38,7 +38,6 @@
 .Sh LIBRARY
 .Lb libc
 .Sh SYNOPSIS
-.In sys/types.h
 .In sys/stat.h
 .Ft int
 .Fn mkdir "const char *path" "mode_t mode"