Soong: libbpf_prog: Define ENABLE_LIBBPF

This flag will be used to switch between legacy support and libbpf
support in android_bpf_defs.h.

Bug: 359646531
Test: Manual/TH
Change-Id: I954986c7bcd3d477a6a59c6632e4db6e74e58f92
Signed-off-by: Neill Kapron <nkapron@google.com>