selftests/bpf: Fix bash reference in Makefile

'|& ...' is a bash 4.0+ construct which is not guaranteed to be available
when using '$(shell ...)' in a Makefile. Fall back to the more portable
'2>&1 | ...'.

Fixes the following warning during compilation:

	/bin/sh: 1: Syntax error: "&" unexpected

Signed-off-by: Joe Stringer <joe@wand.net.nz>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>