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
Merge pull request #6 from matthiasbeyer/fix-issue-#5

Fix issue #5 - compiler warnings about not always being able to inline.