Explicitly include stdint.h for the int64_t definition.

--- plugins/eg-fifths.lv2/fifths.c	2014-02-08 04:03:16.000000000 +0100
+++ plugins/eg-fitths.lv2/fifths.c	2015-10-09 17:20:36.198433090 +0200
@@ -18,6 +18,7 @@
 #include <math.h>
 #include <stdlib.h>
 #include <string.h>
+#include <stdint.h>
 #ifndef __cplusplus
 #    include <stdbool.h>
 #endif
