xkbcomp: fix stack overflow when evaluating boolean negation

The expression evaluator would go into an infinite recursion when
evaluating something like this as a boolean: `!True`. Instead of
recursing to just `True` and negating, it recursed to `!True` itself
again.

Bug inherited from xkbcomp.

Caught with the afl fuzzer.

Signed-off-by: Ran Benita <ran234@gmail.com>