Fix broken mosquitto_passwd on FreeBSD.

Closes #1032. Thanks to jkoopman, Joseph Benden and Ari Suutari.

Bug: https://github.com/eclipse/mosquitto/issues/1032

ChangeLog.txt

@@ -8,6 +8,7 @@ Broker:
 - Make error messages for missing config file clearer.
 - Fix some Coverity Scan reported errors that could occur when the broker was
   already failing to start.
+- Fix broken mosquitto_passwd on FreeBSD. Closes #1032.
 
 Library:
 - Use higher resolution timer for random initialisation of client id

src/mosquitto_passwd.c

@@ -17,6 +17,7 @@ Contributors:
 #include "config.h"
 
 #include <errno.h>
+#include <openssl/opensslv.h>
 #include <openssl/evp.h>
 #include <openssl/rand.h>
 #include <openssl/buffer.h>