Mark Julia headers as SYSTEM to silence warnings from Julia headers

[halleysfifthinc]

Julia 1.12 headers produce compiler warnings about unused parameters (in the
jl_gc_wb_fresh and jl_gc_wb_knownold) functions. These warnings are not useful/relevant
to people building libcxxwrap.

Adding the SYSTEM property to the Julia header include directory tells CMake to silence
warnings from headers in that directory(s).

[barche]

Thanks, I had no idea this option existed!