use the "bool" data type instead of "int"

"bool" should be used in C99 programs for boolean values.

src/decoder_api.c

@@ -85,7 +85,7 @@ double decoder_seek_where(mpd_unused struct decoder * decoder)
 {
 	assert(dc.command == DECODE_COMMAND_SEEK);
 
-	decoder->seeking = 1;
+	decoder->seeking = true;
 
 	return dc.seekWhere;
 }