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>
diff --git a/bpf/libbpf/libbpf_prog.go b/bpf/libbpf/libbpf_prog.go
index 0ca7af1..9cf0786 100644
--- a/bpf/libbpf/libbpf_prog.go
+++ b/bpf/libbpf/libbpf_prog.go
@@ -158,7 +158,8 @@
 		"-Wall",
 		"-Werror",
 		"-Wextra",
-
+		// Flag to assist with the transition to libbpf
+		"-DENABLE_LIBBPF",
 		"-isystem bionic/libc/include",
 		"-isystem bionic/libc/kernel/uapi",
 		// The architecture doesn't matter here, but asm/types.h is included by linux/types.h.