u_int32_t (etc) is a BSD type.  Define _BSD_SOURCE so musl declares it.

--- libtirpc-1.2.5/tirpc/rpc/types.h.old	2019-12-19 18:55:48.000000000 +0000
+++ libtirpc-1.2.5/tirpc/rpc/types.h	2019-12-29 19:17:59.146261005 +0000
@@ -38,6 +38,7 @@
 #ifndef _TIRPC_TYPES_H
 #define _TIRPC_TYPES_H
 
+#define _BSD_SOURCE
 #include <sys/types.h>
 
 typedef int32_t bool_t;