selftests/bpf: Set vpath in Makefile to search for skels

Auto-dependencies generated for %.test.o files refer to skels using
filenames as opposed to full paths. This requires make to be able to
link this name to an actual path, because not all generated skels are
put in the working directory.

In the original patch [1], this was mitigated by this target:

$(notdir %.skel.h): $(TRUNNER_OUTPUT)/%.skel.h
@true

This turned out to be insufficient.

First, %.lskel.h and %.subskel.h were missed, because a typical
selftests/bpf build could find these files in the working directory.
This error was detected by an out-of-tree build [2].

Second, even with missing rules added, this target causes unnecessary
rebuilds in the out-of-tree case, as X.skel.h is searched for in the
working directory, and not in the $(OUTPUT).

Using vpath directive [3] is a better solution. Instead of introducing
a separate target (X.skel.h in addition to $(TRUNNER_OUTPUT)/X.skel.h),
make is instructed to search for skels in the output, which allows make
to correctly detect that skel has already been generated.

[1]: https://lore.kernel.org/bpf/VJihUTnvtwEgv_mOnpfy7EgD9D2MPNoHO-MlANeLIzLJPGhDeyOuGKIYyKgk0O6KPjfM-MuhtvPwZcngN8WFqbTnTRyCSMc2aMZ1ODm1T_g=@pm.me/
[2]: https://lore.kernel.org/bpf/CIjrhJwoIqMc2IhuppVqh4ZtJGbx8kC8rc9PHhAIU6RccnWT4I04F_EIr4GxQwxZe89McuGJlCnUk9UbkdvWtSJjAsd7mHmnTy9F8K2TLZM=@pm.me/
[3]: https://www.gnu.org/software/make/manual/html_node/Selective-Search.html

Reported-by: Björn Töpel <bjorn@kernel.org>
Signed-off-by: Ihor Solodrai <ihor.solodrai@pm.me>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Tested-by: Björn Töpel <bjorn@rivosinc.com>
Link: https://lore.kernel.org/bpf/20240916195919.1872371-2-ihor.solodrai@pm.me
Signed-off-by: Alexei Starovoitov <ast@kernel.org>