Makefile.am: fix linking xiph with ogg

The internal static xiph library needs to link with libogg. Otherwise
building mpd will fail:

```
/mips-linux-gnu/bin/ld: libxiph.a(libxiph_a-OggVisitor.o): undefined
reference to symbol 'ogg_stream_packetout'
```

Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>

Makefile.am

@@ -615,7 +615,8 @@ libxiph_a_SOURCES += \
 	src/lib/xiph/OggStreamState.hxx
 endif
 
-XIPH_LIBS = libxiph.a
+XIPH_LIBS = libxiph.a \
+	$(OGG_LIBS)
 
 endif