tools/power turbostat: Fix gcc build warnings

[ Upstream commit d8d005ba6afa502ca37ced5782f672c4d2fc1515 ]

Warning: ‘__builtin_strncpy’ specified bound 20 equals destination size
	[-Wstringop-truncation]

reduce param to strncpy, to guarantee that a null byte is always copied
into destination buffer.

Signed-off-by: Len Brown <len.brown@intel.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>