Add opus_custom.h to celt_headers.txt.

This file is included unconditionally by celt.h.
As such it's a required part of the build and
should be in the header index files.

celt_headers.txt

@@ -4,6 +4,7 @@ celt/bands.h \
 celt/celt.h \
 include/opus_types.h \
 include/opus_defines.h \
+include/opus_custom.h \
 celt/cwrs.h \
 celt/ecintrin.h \
 celt/entcode.h \

doc/build_draft.sh

@@ -26,7 +26,6 @@ done
 cp -a "${toplevel}"/src/opus_demo.c "${destdir}"/src/
 cp -a "${toplevel}"/src/opus_compare.c "${destdir}"/src/
 cp -a "${toplevel}"/celt/opus_custom_demo.c "${destdir}"/celt/
-cp -a "${toplevel}"/include/opus_custom.h "${destdir}"/celt/
 cp -a "${toplevel}"/Makefile.draft "${destdir}"/Makefile
 cp -a "${toplevel}"/opus_sources.mk "${destdir}"/
 cp -a "${toplevel}"/celt_sources.mk "${destdir}"/