The types uid_t and gid_t are defined in <unistd.h>, which
isn't included as a side effect with musl libc.

--- a/src/utils/io.c	2015-02-10 17:59:34.000000000 +0100
+++ b/src/utils/io.c	2015-08-07 15:07:56.127793677 +0200
@@ -26,6 +26,7 @@
 #include <string.h>
 #include <termios.h>
 #include <stdlib.h>
+#include <unistd.h>
 #include <errno.h>
 #include <sys/mman.h>
 #include "ecryptfs.h"
--- a/src/utils/mount.ecryptfs.c	2015-03-10 17:45:52.000000000 +0100
+++ b/src/utils/mount.ecryptfs.c	2015-08-07 15:15:28.411834229 +0200
@@ -23,6 +23,7 @@
 
 #include <errno.h>
 #include <fcntl.h>
+#include <unistd.h>
 #include <getopt.h>
 #include <keyutils.h>
 #include <stdio.h>