Add new public headers to the install targets.

The new opus_types.h header is required to build against libopus,
so must be installed alongside opus.h. Furthermore, opus-tools
expects access to the multichannel support in opus_multichannel.h.

Makefile.am

@@ -22,7 +22,7 @@ include opus_headers.txt
 
 libopus_la_SOURCES = $(CELT_SOURCES) $(SILK_SOURCES) $(OPUS_SOURCES)
 
-pkginclude_HEADERS = src/opus.h
+pkginclude_HEADERS = src/opus.h src/opus_multistream.h libcelt/opus_types.h
 
 noinst_HEADERS = $(OPUS_HEAD) $(SILK_HEAD) $(CELT_HEAD)