media-tv/mythtv: add subslot dep on libzip

We link against it & it exposes a subslot, so :=.

Bug: https://bugs.gentoo.org/837959
Signed-off-by: Sam James <sam@gentoo.org>

media-tv/mythtv/mythtv-32.0-r1.ebuild

@@ -37,7 +37,7 @@ RDEPEND="
 	acct-user/mythtv
 	dev-libs/glib:2
 	dev-libs/lzo
-	dev-libs/libzip
+	dev-libs/libzip:=
 	dev-qt/qtcore:5
 	dev-qt/qtdbus:5
 	dev-qt/qtgui:5[jpeg]