Add missing headers

fcntl.h in pam_yubico.c is needed on OS X with clang for the build to
succeed, while unistd.h in util.c is required so clang doesn't complain
about implicit declarations of ftruncate and fsync.

pam_yubico.c

@@ -36,6 +36,7 @@
 
 #include <sys/types.h>
 #include <sys/stat.h>
+#include <fcntl.h>
 #include <unistd.h>
 #include <errno.h>
 #include <string.h>

util.c

@@ -36,6 +36,7 @@
 #include <string.h>
 #include <sys/types.h>
 #include <pwd.h>
+#include <unistd.h>
 
 #include "util.h"