perf tools: Do not ignore the default vmlinux.h

The recent change made it possible to generate vmlinux.h from BTF and
to ignore the file.  But we also have a minimal vmlinux.h that will be
used by default.  It should not be ignored by GIT.

Fixes: b7a2d774 ("perf build: Add ability to build with a generated vmlinux.h")
Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Ian Rogers <irogers@google.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202310110451.rvdUZJEY-lkp@intel.com/
Cc: oe-kbuild-all@lists.linux.dev
Signed-off-by: Namhyung Kim <namhyung@kernel.org>

tools/perf/util/bpf_skel/vmlinux/.gitignore

+!vmlinux.h