crosvm: add riscv64-linux to meta.platforms

Theoretically this has been supported for a long time, but last time I
tried it didn't actually build. Now it does.

Changed files
+1
pkgs
by-name
cr
crosvm
+1
pkgs/by-name/cr/crosvm/package.nix
···
license = licenses.bsd3;
platforms = [
"aarch64-linux"
+
"riscv64-linux"
"x86_64-linux"
];
};