media-sound/mixxx: disable building shared libs

Not sure if this is the right approach, but building the ebuild without this
gives a bunch of undefined reference errors to benchmarking code

Better approach might be this + disable benchmarking linking in final shared
build? & disable benchmarking & testing in general when USE=-test

Signed-off-by: Marco Sirabella <marco@sirabella.org>

media-sound/mixxx/mixxx-9999.ebuild

@@ -107,6 +107,7 @@ src_configure() {
 		-DQTKEYCHAIN="$(usex qtkeychain on off)"
 		-DKEYFINDER="$(usex keyfinder on off)"
 		-DDOWNLOAD_MANUAL=OFF
+		-DBUILD_SHARED_LIBS=OFF
 	)
 
 	if [[ "${PV}" == 9999 ]] ; then