KWSys: __int64 and long long may be same type in specialization

For the specialization of hash<>(), the types long long and __int64
may be the same type. While the CMakeLists indicate that if __int64 is
a alias for another type the it will not be enabled, on mingw they
both appear to be the same type and enabled.

This patch only enable specialization for long long OR __int64 to
avoid the potential conflict.

Author: Bradley Lowekamp <blowekamp@mail.nih.gov>
Change-Id: I813a9ac008b296fab5a369c48e6dd5460fd0c035