wireguard: selftests: rename DEBUG_PI_LIST to DEBUG_PLIST

DEBUG_PI_LIST was renamed to DEBUG_PLIST since 8e18faea ("lib/plist:
rename DEBUG_PI_LIST to DEBUG_PLIST").

Signed-off-by: Li Zhijian <lizhijian@cn.fujitsu.com>
Fixes: 8e18faea ("lib/plist: rename DEBUG_PI_LIST to DEBUG_PLIST")
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>