nixos/kexec: Fix typo in meta.platforms

Evaluation error introduced in 599c4df4

.

There is only a "platformS" attribute in kexectools.meta, so let's use
this and from the code in the kexec module it operates on a list,
matching the corresponding platforms, so this seems to be the attribute
the original author intended.

Tested by building nixos/tests/kexec.nix on x86_64-linux and while it
evaluates now, the test still fails by timing out shortly after the
kexec:

machine: waiting for the VM to finish booting
machine# Cannot find the ESP partition mount point.

This however seems to be an unrelated issue and was also the case before
the commit mentioned above.

Signed-off-by: aszlig <aszlig@nix.build>
Cc: @edolstra, @dezgeg