package/opus: enable custom modes

Enable custom modes to avoid the following build failure with jack2
raised since commit 618a23100fe5ff54b90ada1a8f23453f6c5a77dc:

Checking for header opus/opus_custom.h   : not found
The above check failed, but the checkee is required for --opus.

It should be noted that before this commit, opus was silently disabled

Fixes:
 - http://autobuild.buildroot.org/results/b1c050ccd6152c43a6da5f5d2174c3cc0dc2ff3e

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

package/opus/opus.mk

@@ -17,6 +17,7 @@ OPUS_CFLAGS += -O0
 endif
 
 OPUS_CONF_ENV = CFLAGS="$(OPUS_CFLAGS)"
+OPUS_CONF_OPTS = --enable-custom-modes
 
 ifeq ($(BR2_PACKAGE_OPUS_FIXED_POINT),y)
 OPUS_CONF_OPTS += --enable-fixed-point