virtual_oss: Take MK_CUSE into account

Reported by:	brooks
Fixes:		f74f891581 ("src.opts: Introduce MK_SOUND")
Sponsored by:	The FreeBSD Foundation
MFC after:	4 days
Reviewed by:	brooks, emaste
Differential Revision:	https://reviews.freebsd.org/D54708

lib/Makefile

@@ -237,7 +237,7 @@ SUBDIR.${MK_RADIUS_SUPPORT}+=	libradius
 SUBDIR.${MK_SENDMAIL}+=	libmilter libsm libsmdb libsmutil
 SUBDIR.${MK_TELNET}+=	libtelnet
 SUBDIR.${MK_SOUND}+=	libmixer
-SUBDIR.${MK_SOUND}+=	virtual_oss
+SUBDIR.${MK_CUSE}.${MK_SOUND}+=	virtual_oss
 SUBDIR.${MK_TESTS_SUPPORT}+=	atf
 SUBDIR.${MK_TESTS_SUPPORT}+=	liblutok
 SUBDIR.${MK_TESTS}+=	tests

usr.sbin/Makefile

@@ -199,7 +199,7 @@ SUBDIR.${MK_SENDMAIL}+=	praliases
 SUBDIR.${MK_SENDMAIL}+=	sendmail
 SUBDIR.${MK_SOUND}+=	mixer
 SUBDIR.${MK_SOUND}+=	sndctl
-SUBDIR.${MK_SOUND}+=	virtual_oss
+SUBDIR.${MK_CUSE}.${MK_SOUND}+=	virtual_oss
 SUBDIR.${MK_TCP_WRAPPERS}+=	tcpdchk
 SUBDIR.${MK_TCP_WRAPPERS}+=	tcpdmatch
 SUBDIR.${MK_TOOLCHAIN}+=	config