Fix missing header includes for OS X

src/civetweb.c

@@ -90,6 +90,8 @@
 #include <sys/time.h>
 #include <mach/clock.h>
 #include <mach/mach.h>
+#include <mach/mach_time.h>
+#include <assert.h>
 
 //clock_gettime is not implemented on OSX
 int clock_gettime(int clk_id, struct timespec* t) {