selftests/bpf: Copy runqslower to OUTPUT directory

$(OUTPUT)/runqslower makefile target doesn't actually create runqslower
binary in the $(OUTPUT) directory. As lib.mk expects all
TEST_GEN_PROGS_EXTENDED (which runqslower is a part of) to be present in
the OUTPUT directory, this results in an error when running e.g. `make
install`:

rsync: link_stat "tools/testing/selftests/bpf/runqslower" failed: No
       such file or directory (2)

Copy the binary into the OUTPUT directory after building it to fix the
error.

Fixes: 3a0d3092 ("selftests/bpf: Build runqslower from selftests")
Signed-off-by: Veronika Kabatova <vkabatov@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20200428173742.2988395-1-vkabatov@redhat.com

tools/testing/selftests/bpf/Makefile

@@ -141,7 +141,8 @@ VMLINUX_BTF := $(abspath $(firstword $(wildcard $(VMLINUX_BTF_PATHS))))
 $(OUTPUT)/runqslower: $(BPFOBJ)
 	$(Q)$(MAKE) $(submake_extras) -C $(TOOLSDIR)/bpf/runqslower	\
 		    OUTPUT=$(SCRATCH_DIR)/ VMLINUX_BTF=$(VMLINUX_BTF)   \
-		    BPFOBJ=$(BPFOBJ) BPF_INCLUDE=$(INCLUDE_DIR)
+		    BPFOBJ=$(BPFOBJ) BPF_INCLUDE=$(INCLUDE_DIR) &&	\
+		    cp $(SCRATCH_DIR)/runqslower $@
 
 $(TEST_GEN_PROGS) $(TEST_GEN_PROGS_EXTENDED): $(OUTPUT)/test_stub.o $(BPFOBJ)