grsecurity: work around for #20490

In `scripts/Makefile.modinst`, the code that generates the list of
modules to install passes file names via the command line.  When
installing a grsecurity kernel, this list appears to exceed the
shell's argument list limit, as in

    make[2]: execvp: /nix/store/[...]-bash-4.3-p46/bin/bash: Argument list too long

The build does not fail, however, but the list of modules to be installed ends
up being empty.  Thus, the resulting kernel package output contains no modules,
rendering it useless.

We work around this by patching the makefile to use `find -exec` to
process files.  Why this would occur for grsecurity and not other
kernels is unknown, most likely there's something *else* that is
actually causing this behaviour, so this is a temporary fix until that
cause is found.

Fixes https://github.com/NixOS/nixpkgs/issues/20490