Since b2f9cd34e7 ("gobject-introspection: use wrapper.nix for the
native package too so we can propagate the dev output"),
gobject-introspection doesn't need to be included in buildInputs when
it's in nativeBuildInputs, as it's propagated from the setup hook.
Removing the build input fixes evaluation in buildPackages when
cross-compiling to platforms that gobject-introspection is unsupported
on, like ghcjs.
Tested building natively on x86_64-linux, in
pkgsCross.aarch64-multiplatform, and in pkgsCross.ghcjs.buildPackages.
