format_sln: add .slin as supported file extension

Adds '.slin' to existing supported file extensions:
.sln and .raw

ASTERISK-30465

Change-Id: Ice848addc03a64c8404b87cb5d3b13399c57e496
(cherry picked from commit 8d2ffc8aa54315b937712bc725fd813a37e73158)

doc/CHANGES-staging/format_sln_support_for_slin.txt

+Subject: format_sln
+
+format_sln now recognizes '.slin' as a valid
+file extension in addition to the existing
+'.sln' and '.raw'.

formats/format_sln.c

@@ -127,7 +127,7 @@ static off_t slinear_tell(struct ast_filestream *fs)
 static struct ast_frame *slinear_read(struct ast_filestream *s, int *whennext){return generic_read(s, whennext, 320);}
 static struct ast_format_def slin_f = {
 	.name = "sln",
-	.exts = "sln|raw",
+	.exts = "sln|slin|raw",
 	.write = slinear_write,
 	.seek = slinear_seek,
 	.trunc = slinear_trunc,