Fix missing mach/mach_time.h header on OS X.

Closes #1831. Thanks to P-Hagen.

ChangeLog.txt

@@ -1,6 +1,7 @@
 Client library:
 - Fix send quota being incorrecly reset on reconnect. Closes #1822.
 - Don't use logging until log mutex is initialised. Closes #1819.
+- Fix missing mach/mach_time.h header on OS X. Closes #1831.
 
 1.6.12 - 2020-08-19
 ===================

lib/mosquitto.c

@@ -24,6 +24,10 @@ Contributors:
 #include <strings.h>
 #endif
 
+#if defined(__APPLE__)
+#  include <mach/mach_time.h>
+#endif
+
 #include "logging_mosq.h"
 #include "mosquitto.h"
 #include "mosquitto_internal.h"