[5.4.y] selftests/bpf build broken by "bpf: Zero-fill..."

Hi Sasha-

This v5.4.78 commit breaks the tools/testing/selftests/bpf build:

[linux-5.4.y] c602ad2b52dc bpf: Zero-fill re-used per-cpu map element

Like this:

prog_tests/map_init.c:5:10: fatal error: test_map_init.skel.h: No such file or directory 5 | #include "test_map_init.skel.h"

Because tools/testing/selftests/bpf/Makefile in v5.4 does not have the "skeleton header generation" stuff (circa v5.6).

Reverting c602ad2b52dc from linux-5.4.y fixes it.

-Kamal