tools/runqslower: Allow substituting custom vmlinux.h for the build

Just like was done for bpftool and selftests in ec23eb70 ("tools/bpftool:
Allow substituting custom vmlinux.h for the build") and ca4db638
("selftests/bpf: Allow substituting custom vmlinux.h for selftests build"),
allow to provide pre-generated vmlinux.h for runqslower build.
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20210303004010.653954-1-andrii@kernel.org

tools/bpf/runqslower/Makefile

@@ -69,12 +69,16 @@ $(OUTPUT) $(BPFOBJ_OUTPUT) $(BPFTOOL_OUTPUT):
 	$(QUIET_MKDIR)mkdir -p $@
 
 $(OUTPUT)/vmlinux.h: $(VMLINUX_BTF_PATH) | $(OUTPUT) $(BPFTOOL)
+ifeq ($(VMLINUX_H),)
 	$(Q)if [ ! -e "$(VMLINUX_BTF_PATH)" ] ; then \
 		echo "Couldn't find kernel BTF; set VMLINUX_BTF to"	       \
 			"specify its location." >&2;			       \
 		exit 1;\
 	fi
 	$(QUIET_GEN)$(BPFTOOL) btf dump file $(VMLINUX_BTF_PATH) format c > $@
+else
+	$(Q)cp "$(VMLINUX_H)" $@
+endif
 
 $(BPFOBJ): $(wildcard $(LIBBPF_SRC)/*.[ch] $(LIBBPF_SRC)/Makefile) | $(BPFOBJ_OUTPUT)
 	$(Q)$(MAKE) $(submake_extras) -C $(LIBBPF_SRC) OUTPUT=$(BPFOBJ_OUTPUT) $@