build: fix kernel 5.4 on macos

As part of the kernel build process there are utilities built to run on
the host system that expect linux kernel headers to be available.
Unfortunately macos/darwin doesn't have these headers.

vdso2c requires types.h so provide a minimal stub to satisfy it.

Signed-off-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>