format_gsm.c: Added mime type

Sometimes it's impossible to get a file extension from URL
(eg. http://example.com/gsm/your) so we have to rely on content-type header.
Currenly, asterisk does not support content-type for gsm format(unlike wav).
Added audio/gsm according to https://www.rfc-editor.org/rfc/rfc4856.html

(cherry picked from commit ced3b41327)

formats/format_gsm.c

@@ -174,6 +174,7 @@ static off_t gsm_tell(struct ast_filestream *fs)
 static struct ast_format_def gsm_f = {
 	.name = "gsm",
 	.exts = "gsm",
+	.mime_types = "audio/gsm",
 	.write = gsm_write,
 	.seek =	gsm_seek,
 	.trunc = gsm_trunc,