Move MK_REPRODUCIBLE_PATHS to bsd.opts.mk

In commit faeaa25f5624 MK_REPRODUCIBLE_PATHS was introduced, but some
ports that include bsd.debug.mk then started to fail with:

make: /usr/share/mk/bsd.debug.mk:19: Variable "MK_REPRODUCIBLE_PATHS" is undefined
        in /usr/share/mk/bsd.lib.mk:525
        in /wrkdirs/usr/ports/devel/libsysinfo/work/libsysinfo-0.0.3/Makefile:16
make: Fatal errors encountered -- cannot continue

This is because bsd.debug.mk includes bsd.opts.mk, not src.opts.mk. Move
the MK_REPRODUCIBLE_PATHS option to bsd.opts.mk to fix this.

Reviewed by: imp, emaste
Fixes: faeaa25f5624