rust: fix cfgs of proc-macro2 for 1.63.0

Replay the configuration that would be computed by build.rs when compiling
on a 1.63.0 compiler.

Reviewed-by: Junjie Mao <junjie.mao@hotmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
1 file changed