libcompat: avoid installing include files twice

Previously some headers were getting installed twice, once as expected
and then a second time as part of the compat32 library stage.

Makefile.libcompat sets -DLIBRARIES_ONLY for the install make invocation
which causes bsd.lib.mk to skip headers. However some headers are
handled via bsd.prog.mk, which does not use LIBRARIES_ONLY. Explicitly
set MK_INCLUDES=no.

Reviewed by: brooks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D37413