bpf: Missing trailing slash in tools/testing/selftests/bpf/Makefile

tools/lib/bpf/Makefile assumes that the patch in OUTPUT is a directory
and that it includes a trailing slash.  This seems to be a common
expectation for OUTPUT among all the Makefiles.

In the rule for runqslower in tools/testing/selftests/bpf/Makefile the
variable BPFTOOL_OUTPUT is set to a directory name that lacks a
trailing slash.  This results in a malformed BPF_HELPER_DEFS being
defined in lib/bpf/Makefile.

This problem becomes evident when a file like
tools/lib/bpf/bpf_tracing.h gets updated.

This patch fixes the problem by adding the missing slash in the value
for BPFTOOL_OUTPUT in the $(OUTPUT)/runqslower rule.

Regtested by running selftests in bpf-next master and building
samples/bpf programs.

Signed-off-by: Jose E. Marchesi <jose.marchesi@oracle.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20240502140831.23915-1-jose.marchesi@oracle.com