fix return type of getopt_long

On some platforms this manifested as an inability to parse command line
arguments and login.  Fixes:

lpass.c:58:2: warning: comparison is always true due to limited range of
data type [-Wtype-limits]
  while ((option = getopt_long(argc, argv, "vh", long_options,
&option_index)) != -1) {
  ^
Signed-off-by: Bob Copeland <copeland@lastpass.com>