perf bpf: Check that the minimal vmlinux.h installed is the latest one

When building BPF skels perf will, by default, install a minimalistic
vmlinux.h file with the types needed by the BPF skels in
tools/perf/util/bpf_skel/ in its build directory.

When 29d16de2 ("perf augmented_raw_syscalls.bpf: Move 'struct
timespec64' to vmlinux.h") was added, a type used in the augmented_raw_syscalls
BPF skel, 'struct timespec64' was not found when building from a pre-existing
build directory, because the vmlinux.h there didn't contain that type,
ending up with this error, spotted in linux-next:

    CLANG   /tmp/build/perf-tools-next/util/bpf_skel/.tmp/augmented_raw_syscalls.bpf.o
  util/bpf_skel/augmented_raw_syscalls.bpf.c:329:15: error: invalid application of 'sizeof' to an incomplete type 'struct timespec64'
    329 |         __u32 size = sizeof(struct timespec64);
        |                      ^     ~~~~~~~~~~~~~~~~~~~
  util/bpf_skel/augmented_raw_syscalls.bpf.c:329:29: note: forward declaration of 'struct timespec64'
    329 |         __u32 size = sizeof(struct timespec64);
        |                                    ^
  util/bpf_skel/augmented_raw_syscalls.bpf.c:350:15: error: invalid application of 'sizeof' to an incomplete type 'struct timespec64'
    350 |         __u32 size = sizeof(struct timespec64);
        |                      ^     ~~~~~~~~~~~~~~~~~~~
  util/bpf_skel/augmented_raw_syscalls.bpf.c:350:29: note: forward declaration of 'struct timespec64'
    350 |         __u32 size = sizeof(struct timespec64);
        |                                    ^
  2 errors generated.
  make[2]: *** [Makefile.perf:1158: /tmp/build/perf-tools-next/util/bpf_skel/.tmp/augmented_raw_syscalls.bpf.o] Error 1
  make[2]: *** Waiting for unfinished jobs....
  make[1]: *** [Makefile.perf:261: sub-make] Error 2
  make: *** [Makefile:113: install-bin] Error 2
  make: Leaving directory '/home/acme/git/perf-tools-next/tools/perf'

So add a Makefile dependency (Namhyung's suggestion) to make sure that
the new tools/perf/util/bpf_skel/vmlinux/vmlinux.h minimal vmlinux is
updated in the build directory, providing the moved 'struct timespec64'
type.

Fixes: 29d16de2

 ("perf augmented_raw_syscalls.bpf: Move 'struct timespec64' to vmlinux.h")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Reviewed-by: Ian Rogers <irogers@google.com>
Suggested-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/ZdoPrWg-qYFpBJbz@x1