kmod.mk: Don't split out debug symbols if requested

Ports bsd.kmod.mk explicitly sets MK_KERNEL_SYMBOLS=no to prevent auto-
splitting of debuginfo from kernel modules. If that knob is set, don't
split out a .ko.debug and .ko from .ko.full; just generate a .ko with
debuginfo and leave it be.

Otherwise, with DEBUG_FLAGS set and MK_KERNEL_SYMBOLS=no, we would helpfully
strip out the debuginfo from the .ko.full and then not install it. That is
not the desired result a WITH_DEBUG port kmod build.

Reviewed by: emaste, jhb
Differential Revision: https://reviews.freebsd.org/D24835