Makefile.am: fix build failure when libxiph.a is not used

Some "ar" implementations don't allow empty archives, and failed to
build it.  The real reason (which is hidden with GNU binutils) is that
our Makefile.am was incorrect, and didn't completely disable
libxiph.a.

See https://bugs.musicpd.org/view.php?id=4526

Makefile.am

@@ -20,7 +20,6 @@ noinst_LIBRARIES = \
 	libevent.a \
 	libicu.a \
 	libpcm.a \
-	libxiph.a \
 	libconf.a \
 	libtag.a \
 	libinput.a \
@@ -1829,7 +1828,7 @@ test_dump_playlist_LDADD = \
 	$(FLAC_LIBS) \
 	$(DECODER_LIBS) \
 	$(TAG_LIBS) \
-	libxiph.a \
+	$(XIPH_LIBS) \
 	$(INPUT_LIBS) \
 	$(ARCHIVE_LIBS) \
 	libconf.a \