Fix warning of strncpy() size field.

bcc/tests/cc/test_c_api.cc:159:10: warning: ‘char* strncpy(char*, const char*, size_t)’ specified bound 1024 equals destination size [-Wstringop-truncation]
  159 |   strncpy(libpath, lm->l_name, 1024);
      |   ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~

tests/cc/test_c_api.cc

@@ -156,7 +156,7 @@ static int mntns_func(void *arg) {
  return -1;
  }
 
- strncpy(libpath, lm->l_name, 1024);
+ strncpy(libpath, lm->l_name, sizeof(libpath) - 1);
  dlclose(dlhdl);
  dlhdl = NULL;