selftests/bpf: Fix out-of-tree build

When building out-of-tree, the .skel.h files are generated into the
$(OUTPUT) directory, rather than $(CURDIR). Add $(OUTPUT) to the include
paths.

Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210113163319.1516382-3-jean-philippe@linaro.org