linux: use correct conditional for wireguard kernel config fixup

Commit de591c5c (package/wireguard-linux-compat: new package) split up
the wireguard package in wireguard-tools and wireguard-linux-compat, but
forgot to update the conditional in linux.mk, so the kernel config fixups
needed for wireguard are no longer applied.

Update the conditional to use the BR2_PACKAGE_WIREGUARD_LINUX_COMPAT symbol
instead.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>