libsamplerate: Only build when virtual_oss is built

Gate libsamplerate behind MK_CUSE and MK_SOUND, like virtual_oss.
Nothing else uses this library, so there's no point building it
if we aren't building virtual_oss.

This avoids building a useless FreeBSD-sound package containing
only this library when WITHOUT_SOUND is set.

MFC after:	1 week
Reviewed by:	christos, emaste
Differential Revision:	https://reviews.freebsd.org/D56164
Sponsored by:		https://www.patreon.com/bsdivy

lib/Makefile

@@ -91,7 +91,6 @@ SUBDIR=	${SUBDIR_BOOTSTRAP} \
 	librss \
 	librt \
 	librtld_db \
-	libsamplerate \
 	libsbuf \
 	libsmb \
 	libstdbuf \
@@ -237,7 +236,7 @@ SUBDIR.${MK_RADIUS_SUPPORT}+=	libradius
 SUBDIR.${MK_SENDMAIL}+=	libmilter libsm libsmdb libsmutil
 SUBDIR.${MK_TELNET}+=	libtelnet
 SUBDIR.${MK_SOUND}+=	libmixer
-SUBDIR.${MK_CUSE}.${MK_SOUND}+=	virtual_oss
+SUBDIR.${MK_CUSE}.${MK_SOUND}+=	libsamplerate virtual_oss
 SUBDIR.${MK_TESTS_SUPPORT}+=	atf
 SUBDIR.${MK_TESTS_SUPPORT}+=	liblutok
 SUBDIR.${MK_TESTS}+=	tests