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

Makefile.libcompat

@@ -36,6 +36,7 @@ LIBCOMPATWMAKE+=	${TIME_ENV} ${LIBCOMPATWMAKEENV} ${MAKE} ${LIBCOMPATWMAKEFLAGS}
 			MK_MAN=no MK_HTML=no
 LIBCOMPATIMAKE+=	${LIBCOMPATWMAKE:NINSTALL=*:NDESTDIR=*} \
 			${IMAKE_INSTALL} \
+			MK_INCLUDES=no \
 			-DLIBRARIES_ONLY
 
 _LC_LIBDIRS.yes=		lib