psppire-axis: Use rint() instead of nearbyint(), for portability.

psppire-axis: Use rint() instead of nearbyint(), for portability.

rint() and nearbyint() are identical except that nearbyint() will not
raise the inexact result exception, which we don't care about.  OpenBSD
and probably other OSes have rint() but not nearbyint().

Perhaps a better fix would be to add a nearbyint module to Gnulib.