tools/runqslower: Enable out-of-tree build

Enable out-of-tree build for runqslower. Only set OUTPUT=.output if it
wasn't already set by the user.

Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20201110164310.2600671-6-jean-philippe@linaro.org