teach file.c to recognise AAC audio files.

Reference: /n/sources/patch/applied/file-aac-audio
Date: Thu Aug 22 12:31:31 CES 2013
Signed-off-by: steve@quintile.net

--- /sys/src/cmd/file.c	Thu Aug 22 12:26:24 2013
+++ /sys/src/cmd/file.c	Thu Aug 22 12:26:05 2013
@@ -538,6 +538,7 @@
 	070707,		0xFFFF,		"cpio archive\n", OCTET,
 	0x2F7,		0xFFFF,		"tex dvi\n", "application/dvi",
 	0xfaff,		0xfeff,		"mp3 audio\n",	"audio/mpeg",
+	0xf0ff,		0xf6ff,		"aac audio\n",	"audio/mpeg",
 	0xfeff0000,	0xffffffff,	"utf-32be\n",	"text/plain charset=utf-32be",
 	0xfffe,		0xffffffff,	"utf-32le\n",	"text/plain charset=utf-32le",
 	0xfeff,		0xffff,		"utf-16be\n",	"text/plain charset=utf-16be",