fix mingw-win32 specific warnings in libdivecomputer.c

fix mingw-win32 specific warnings in libdivecomputer.c

1) since %lld is not defined in the MSVC runtime, use
the portable PRId64 macro from inttypes.h for 64bit integers

notice in inttypes.h from mingw-win32:
/* 7.8.1 Macros for format specifiers
 *
 * MS runtime does not yet understand C9x standard "ll"
 * length specifier. It appears to treat "ll" as "l".
 * The non-standard I64 length specifier causes warning in GCC,
 * but understood by MS runtime functions.
 */

2) include unistd.h to disable warning:
warning: implicit declaration of function 'usleep'

Lubomir's code then caused a warning building natively under Linux, which
I fixed as well.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Signed-off-by: Lubomir I. Ivanov <neolit123@gmail.com>