selftests/bpf: Add runqslower binary to .gitignore

With recent changes, runqslower is being copied into selftests/bpf root
directory. So add it into .gitignore.

Fixes: b26d1e2b ("selftests/bpf: Copy runqslower to OUTPUT directory")
Signed-off-by: NAndrii Nakryiko <andriin@fb.com>
Signed-off-by: NAlexei Starovoitov <ast@kernel.org>
Cc: Veronika Kabatova <vkabatov@redhat.com>
Link: https://lore.kernel.org/bpf/20200429012111.277390-12-andriin@fb.com

tools/testing/selftests/bpf/.gitignore

@@ -37,4 +37,4 @@ test_cpp
 /no_alu32
 /bpf_gcc
 /tools
-
+/runqslower