perf help: Remove needless use of strncpy()

perf help: Remove needless use of strncpy()

Since we make sure the destination buffer has at least strlen(orig) + 1,
no need to do a strncpy(dest, orig, strlen(orig)), just use strcpy(dest,
orig).

This silences this gcc 8.2 warning on Alpine Linux:

  In function 'add_man_viewer',
      inlined from 'perf_help_config' at builtin-help.c:284:3:
  builtin-help.c:192:2: error: 'strncpy' output truncated before terminating nul copying as many bytes from a string as its length [-Werror=stringop-truncation]
    strncpy((*p)->name, name, len);
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  builtin-help.c: In function 'perf_help_config':
  builtin-help.c:187:15: note: length computed here
    size_t len = strlen(name);
                 ^~~~~~~~~~~~

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Fixes: 078006012401 ("perf_counter tools: add in basic glue from Git")
Link: https://lkml.kernel.org/n/tip-2f69l7drca427ob4km8i7kvo@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>