LibC: Add timegm()

timegm() is like mktime() in that it converts a struct tm to
a timestamp, but it treats the struct tm as UTC instead of as
local time.

timegm() is nonstandard, but availabe in both Linux and BSD,
and it's a useful function to have.

Libraries/LibC/time.h

@@ -54,6 +54,7 @@ typedef int64_t time_t;
 struct tm* localtime(const time_t*);
 struct tm* gmtime(const time_t*);
 time_t mktime(struct tm*);
+time_t timegm(struct tm*);
 time_t time(time_t*);
 char* ctime(const time_t*);
 void tzset();