Fix issue #5

As @larsendt mentioned in #5 [0], we can fix this issue by adding the
inline keyword to the macro.

From [1]:

    The '__attribute__((always_inline))' does not strictly imply
    'inline'.  Newer versions of gcc detect this misuse and issue
    the following warning.  Including the missing 'inline' resolves
    the build warning.

After adding the inline keyword (as in [1]), the lib builds fine for me.

[0]: https://github.com/larsendt/hashtable/issues/5
[1]: https://github.com/zfsonlinux/zfs/commit/10be533e3344f523e1b8d6ab4f0658897a95ac02