`wikiheaders.pl` generates perl warnings on macOS

[icculus]

This is bike shedding, but I see this warning message when generating the SDL man pages:

 Use of uninitialized value $brief in string at /var/folders/3m/p59k4qdj0f17st0gn2cmj3640000gn/T/setupsdl/66c57facf1111a4ec08d3d1abdf3c87f3062ffbea8d9eb4c9c412e9b5e7f59b7/source/build-scripts/wikiheaders.pl line 1902.
 Use of uninitialized value $brief in string at /var/folders/3m/p59k4qdj0f17st0gn2cmj3640000gn/T/setupsdl/66c57facf1111a4ec08d3d1abdf3c87f3062ffbea8d9eb4c9c412e9b5e7f59b7/source/build-scripts/wikiheaders.pl line 1902.

ci link

Originally posted by @madebr in https://github.com/libsdl-org/SDL/issues/9440#issuecomment-2089399963

(this wasn't bikeshedding, fwiw. :P --ryan.)