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 <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Fixes: 078006012401 ("perf_counter tools: add in basic glue from Git")
Link: https://lkml.kernel.org/n/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/builtin-help.c

···
 	while (*p)
 		p = &((*p)->next);
 	*p = zalloc(sizeof(**p) + len + 1);
-	strncpy((*p)->name, name, len);
+	strcpy((*p)->name, name);
 }
 
 static int supported_man_viewer(const char *name, size_t len)