math/nauty: fix build on powerpc

Works around the clang crash, which seems to be fixed in the new snapshots:
PLEASE submit a bug report to https://bugs.freebsd.org/submit/ and include the crash backtrace, preprocessed source, and associated run script.
Stack dump:
0. Program arguments: clang -cc1 -triple powerpc-unknown-freebsd14.2 -emit-obj -O2 -vectorize-loops gutil2-502595.c

1. <eof> parser at end of file
2. Optimizer

Segmentation fault (core dumped)