selftests/bpf: Add host-tools to gitignore

Shows up when cross-compiling:

HOST_SCRATCH_DIR        := $(OUTPUT)/host-tools

vs

SCRATCH_DIR := $(OUTPUT)/tools
HOST_SCRATCH_DIR        := $(SCRATCH_DIR)

Reported-by: John Sperbeck <jsperbeck@google.com>
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20221222213958.2302320-1-sdf@google.com