bpf: fix whitespace for ENCAP_L2 defines in bpf.h

replace tab after #define with space in line with rest of definitions

Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>