logging: Format the timestamp using unsigned year, month, etc.

[moreati]

This fixes a compiler warning about the buffer length

$ go version
go version go1.13.8 linux/amd64
$ go build
...
duk_logging.c: In function ‘duk__logger_prototype_log_shared’:
duk_logging.c:184:64: warning: ‘Z’ directive writing 1 byte into a
region of size between 0 and 9 [-Wformat-overflow=]
184 |  sprintf((char *) date_buf,
"%04d-%02d-%02dT%02d:%02d:%02d.%03dZ",
      |                                                                ^
In file included from /usr/include/stdio.h:867,
                 from duk_logging.c:5:
/usr/include/x86_64-linux-gnu/bits/stdio2.h:36:10: note:
‘__builtin___sprintf_chk’ output between 25 and 85 bytes into a
destination of size 32
   36 |   return __builtin___sprintf_chk (__s, __USE_FORTIFY_LEVEL - 1,
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   37 |       __bos (__s), __fmt, __va_arg_pack ());
      |       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

[pytlesk4]

Thank you!