selftests/bpf: Fix missing space error

Add the missing space after 'dest' variable assignment.
This change will resolve the following checkpatch.pl
script error:

ERROR: spaces required around that '+=' (ctx:VxW)

Signed-off-by: Roberto Valenzuela <valenzuelarober@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20230113180257.39769-1-valenzuelarober@gmail.com