Include makewhatis in ITOOLS when MK_MAN_UTILS is true

Previously it was conditional on MK_MAN. It's possible to build
FreeBSD with man pages but without man page tools. MK_MAN_UTILS
is the conditional used in share/man/Makefile for determining whether
makewhatis is executed at install time, so it is the proper one for
ITOOLS as well.

PR: 210142
MFC after: 1 week