perf svghelper: Fix unchecked usage of strncpy()

perf svghelper: Fix unchecked usage of strncpy()

The strncpy() function may leave the destination string buffer
unterminated, better use strlcpy() that we have a __weak fallback
implementation for systems without it.

In this specific case this would only happen if fgets() was buggy, as
its man page states that it should read one less byte than the size of
the destination buffer, so that it can put the nul byte at the end of
it, so it would never copy 255 non-nul chars, as fgets reads into the
orig buffer at most 254 non-nul chars and terminates it. But lets just
switch to strlcpy to keep the original intent and silence the gcc 8.2
warning.

This fixes this warning on an Alpine Linux Edge system with gcc 8.2:

  In function 'cpu_model',
      inlined from 'svg_cpu_box' at util/svghelper.c:378:2:
  util/svghelper.c:337:5: error: 'strncpy' output may be truncated copying 255 bytes from a string of length 255 [-Werror=stringop-truncation]
       strncpy(cpu_m, &buf[13], 255);
       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Fixes: f48d55ce7871 ("perf: Add a SVG helper library file")
Link: https://lkml.kernel.org/n/tip-xzkoo0gyr56gej39ltivuh9g@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>