selftests/powerpc: Fix build break caused by EXPORT_SYMBOL changes

The changes to make EXPORT_SYMBOL work in asm, specifically commit
9445aa1a

 ("ppc: move exports to definitions"), in the kbuild tree,
breaks some of our selftests.

That is because we symlink the kernel code into the selftest, and shim
the required headers, and we are now missing asm/export.h

So create a minimal export.h to keep the tests building once powerpc and
the kbuild trees are merged.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>