sockstat: Fix build reproducibility

Both usr.bin/sockstat and usr.bin/sockstat/tests build the file
src/usr.bin/sockstat/sockstat.c. Unfortunately, they place the
resulting object file in the same

sockstat: Fix build reproducibility

Both usr.bin/sockstat and usr.bin/sockstat/tests build the file
src/usr.bin/sockstat/sockstat.c. Unfortunately, they place the
resulting object file in the same place, but produce different
files due to paths appearing in the .debug files; this results
in different binaries depending on how the race between the two
builds ends.

Use .PATH to tell make to find sockstat.c in the parent directory
but place the object file in the current object directory, rather
than placing '..' into the name of the source file.

Suggested by: bdrewery, emaste, jrtc27
MFC after: 5 minutes (15.0 build fix)
Sponsored by: https://www.patreon.com/cperciva
Differential Revision: https://reviews.freebsd.org/D53032

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