When multiple `-f FILTER` options were specified, `qmk find` did not
return anything at all instead of printing the list of entries that
matched all of the specified filters.
The problem was that the statement in `_filter_keymap_targets()` that
filled `targets` had a wrong indent and therefore was executed for every
filter instead of only once after applying all filters, and
`valid_keymaps` was actually an iterator and therefore could be used
only once. Moving the statement outside of the loop fixes the problem.
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