nut plugin: fix warning

nut plugin: fix warning

  CC       src/nut_la-nut.lo
In function ‘nut_ca_path’,
    inlined from ‘nut_config.part.1’ at src/nut.c:181:12,
    inlined from ‘nut_config’:
src/nut.c:148:5: warning: ‘strncpy’ specified bound depends on the length of the source argument [-Wstringop-overflow=]
     strncpy(ca_path, value, (strlen(value) + 1));
     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
src/nut.c: In function ‘nut_config’:
src/nut.c:148:30: note: length computed here
     strncpy(ca_path, value, (strlen(value) + 1));
                              ^~~~~~~~~~~~~
  CCLD     nut.la