Make mux_ts a plugin too.

Afterall, ts is a plugin, and is probably as useful if not more.

configure.ac

@@ -1905,7 +1905,7 @@ then
       AC_CHECK_HEADERS(dvbpsi/dr.h,
         [ VLC_ADD_PLUGINS([ts])
           if test "${enable_sout}" != "no"; then
-            VLC_ADD_BUILTINS([mux_ts])
+            VLC_ADD_PLUGINS([mux_ts])
           fi
           VLC_ADD_LDFLAGS([mux_ts ts dvb],[-ldvbpsi]) ],
         [  AC_MSG_WARN([cannot find libdvbpsi headers]) ],