graphics/libjxl: unbreak pkg-config support

Convert static libhwy from package to LDFLAGS, so pkgconf doesn't try
to expand missing package in Requires.private outside of --static.

$ pkg-config --cflags libjxl
Package libhwy was not found in the pkg-config search path.
Perhaps you should add the directory containing `libhwy.pc'
to the PKG_CONFIG_PATH environment variable
Package 'libhwy', required by 'libjxl', not found

PR: 261003
Reported by: Greg V