MAINT: fix warning about visibility tag on clang

[ngoldbaum]

This fixes a warning reported by clang:

In file included from ../numpy/fft/_pocketfft_umath.c:23:
../numpy/fft/pocketfft/pocketfft.h:19:1: warning: attribute 'visibility' is ignored, place it after "struct" to apply attribute to type declaration [-Wignored-attributes]
NPY_VISIBILITY_HIDDEN struct cfft_plan_i;
^

I don't have gcc locally so let's see if other compilers complain

[seberg]

Ooops, I don't think the struct needs it at all? It is just a definition for the later typedef after all, not an actual symbol.

[seberg]

Just adding suggestions, if you want just adopt that. In any case, please feel free to merge once you are happy.