on error, remind user of variable used to set src tree location

trying to build kmod ports without a src tree in /usr/src results in a
failure reporitng that the build requires kernel source files in /usr/src.
Mention also the make variable (SRC_BASE) that sets the path as a hint to
users building against a src tree located elsewhere.

Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D22707