Fix build error on MSVC

[cgohlke]

Fixes the following syntax errors when building with Microsoft Visual C compiler:

src/bitshuffle_core.c(52): error C2061: syntax error: identifier '__attribute__'
src/bitshuffle_core.c(52): error C2059: syntax error: ';'

Re #161.

[kiyo-masui]

Passes the CI which is good.

I'm not very familiar with what the aliasing rules and optimizations actually do, but from a quick search it does seem like changes in behavior are possible (its not just speed). So just to confirm, do you understand what this change actually does, and that you are confident that it will not lead to incorrect behavior on MVC? Obviously this shouldn't change gcc compilations, as confirmed by the CI, but the CI does not cover MVC.

[cgohlke]

My understanding of #161 is that it only affects the NEON code path (not used on Windows) and the SSE code path previously used uint16_t anyway. I'm using this patch in the vendored version of bitshuffle in the imagecodecs package and have not noticed any test failures, even on win-arm64.

[kiyo-masui]

Fair enough!