Solve type conflict preventing the pip wheel to build correctly

[emaballarin]

As per subject, this PR tries to fix the following error

src/rlhafnian.c: At top level:
src/rlhafnian.c:405:8: error: conflicting types for ‘hafnian_loops’
double hafnian_loops(double *mat, int n)
^~~~~~~~~~~~~
In file included from src/rlhafnian.c:14:
src/rlhafnian.h:33:7: note: previous declaration of ‘hafnian_loops’ was here
telem hafnian_loops(telem *mat, int n);

arising when trying to build the pip wheel (i.e.: make wheel) or installing the package directly from PyPi or git repository.

The error, somehow, falls through CI testing (though still being present in the logfiles).

According to the testsuite, and some specific ad-hoc testing, it shouldn't break things.

[nquesada]

Thanks!