config: fix Unicode BOM detection

Defining the BOM as a string makes the array include the
NUL-terminator, which means that the memcpy is going to check for that
as well and thus never match for a nonempty file.

Define the array as three chars, which makes the size correct.

src/config_file.c

@@ -820,7 +820,7 @@ fail_parse:
 
 static int skip_bom(diskfile_backend *cfg)
 {
-	static const char utf8_bom[] = "\xef\xbb\xbf";
+	static const char utf8_bom[] = { '\xef', '\xbb', '\xbf' };
 
 	if (cfg->reader.buffer.size < sizeof(utf8_bom))
 		return 0;