Make C++ happy.

C++ has a stronger type system than C and will not implicitly convert
int's to enum's. If we do not want to give any flags to xkb_context_new,
we must pass 0 as its argument.

Therefore, we create a valid xkb_context_flags enum 0 value to avoid ugly
casts in C++.

Signed-off-by: Wander Lairson Costa <wander.lairson@gmail.com>