Function prototypes don't match functions

[smangham]

Small issue: A 'naive' clone and compile of ks_spectrum on desktop Ubuntu fails, as w_open_ks_eigen is prototyped as includes/io_ks_eigen.h:133 as:

ks_eigen_file *w_open_ks_eigen(int flag, char *filename, int parity);

But defined in generic_ks/io_helpers_ks_eigen.c:82 as:

static ks_eigen_file *w_open_ks_eigen(int flag, char *filename, int parity) {
  ; ...
}

Looking around, it seems that this was allowed by gcc 3, but gcc 4 is stricter. I appreciate that this is almost certainly going to be compiled with older/stable HPC specialised compiler versions, so presumably they aren't complaining about this yet. Is it likely that a fix would have knock-on effects? If not, I'm happy to open a pull request to fix the various instances of it.