perf bpf: Add unistd.h to the headers accessible to bpf proggies

Start with a getpid() function wrapping BPF_FUNC_get_current_pid_tgid,
idea is to mimic the system headers.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: https://lkml.kernel.org/n/tip-zo8hv22onidep7tm785dzxfk@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>