Remove second #include <stdint.h> in util.h

The old system might have actually been better, HOWEVER currently the only supported toolchain known to not ship with stdint is MSVC prior to VS2010 -- So the complicated way of doing it may be a bit silly.

Nevertheless, I've removed the second inclusion of stdint.h, and also define WEBVTT_HAVE_STDINT to '1' if we expect it to be present. This should get the typedefing working correctly.