Revert "bpfilter: Fix build error with CONFIG_BPFILTER_UMH"

This reverts commit 1d273fcc2c29343e59658276b77b02e5897a3123.

Alexei points out there's nothing implying headers will be built
and therefore exist under usr/include, so this fix doesn't make
much sense.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>