devel/qjson: Improve clang 16 workaround for usage of the 'register' keyword

Commit 3237ce1557467a3f89a3d1fa7a9308161665bf0c fixed the build on
CURRENT with a heavy hammer that made register a macro. This change
passes -Wno-register to the compiler instead, which is cleaner.

Approved by: arrowd
Differential Revision: https://reviews.freebsd.org/D40938