string.c: fix compilation warning
string.c: fix compilation warning

src/string.c: In function 'ndm_string_dup':
src/string.c:11:3: error: 'strncpy' specified bound depends on the length of
the source argument [-Werror=stringop-overflow=]
   strncpy(s_copy, s, buf_size);
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
src/string.c:7:26: note: length computed here
  const size_t buf_size = strlen(s) + 1;
                          ^~~~~~~~~

ndmsystems/libndm