bpftool: Fix make dependencies for vmlinux.h

With pre-generated vmlinux.h there is no dependency on neither vmlinux
nor bootstrap bpftool. Define dependencies separately for both modes.
This avoids needless rebuilds in some corner cases.

Suggested-by: Jan Stancek <jstancek@redhat.com>
Signed-off-by: Artem Savkov <asavkov@redhat.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Quentin Monnet <qmo@kernel.org>
Link: https://lore.kernel.org/bpf/20240513112658.43691-1-asavkov@redhat.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/bpf/bpftool/Makefile

@@ -204,10 +204,11 @@ ifeq ($(feature-clang-bpf-co-re),1)
 
 BUILD_BPF_SKELS := 1
 
-$(OUTPUT)vmlinux.h: $(VMLINUX_BTF) $(BPFTOOL_BOOTSTRAP)
 ifeq ($(VMLINUX_H),)
+$(OUTPUT)vmlinux.h: $(VMLINUX_BTF) $(BPFTOOL_BOOTSTRAP)
 	$(QUIET_GEN)$(BPFTOOL_BOOTSTRAP) btf dump file $< format c > $@
 else
+$(OUTPUT)vmlinux.h: $(VMLINUX_H)
 	$(Q)cp "$(VMLINUX_H)" $@
 endif