MAINTAINERS: Add include/hw/audio/ to audio section

audio.h and pcspk.h are recognized as maintained files now.
Signed-off-by: NThomas Huth <thuth@redhat.com>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

MAINTAINERS

@@ -1046,6 +1046,7 @@ M: Gerd Hoffmann <kraxel@redhat.com>
 S: Maintained
 F: audio/
 F: hw/audio/
+F: include/hw/audio/
 F: tests/ac97-test.c
 F: tests/es1370-test.c
 F: tests/intel-hda-test.c