perf beauty socket: Rename header_dir to uapi_header_dir

Paving the way to pass more headers to be consumed, like
tools/perf/trace/beauty/include/linux/socket.h in addition to the
current tools/include/uapi/linux/in.h, to get the SOL_* defines.

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