Resolve Mac OS X 10.9 compilation warning: -Wmissing-prototypes for clock_gettime

src/civetweb.c

@@ -127,6 +127,8 @@ mg_static_assert(sizeof(void *) >= sizeof(int), "data type size check");
 #include <assert.h>
 
 /* clock_gettime is not implemented on OSX */
+int clock_gettime(int clk_id, struct timespec *t);
+
 int clock_gettime(int clk_id, struct timespec *t)
 {
 	if (clk_id == CLOCK_REALTIME) {