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