perf tools bpf: Add vmlinux.h to .gitignore

Now that BPF skel based tools will be built by default if the toolchain
pieces that are needed are available, building directly on the source
tree will produce a vmlinux.h from the BTF info that needs to get
ignored.

Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>