chore(signal_processing): add a code owner

[takayuki5168]

Description

add a code owner for the signal_processing package.

Tests performed

No need

Effects on system behavior

Nothing

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• I've confirmed the contribution guidelines.
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In-review checklist for the PR reviewers

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Post-review checklist for the PR author

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• There are no open discussions or they are tracked via tickets.

After all checkboxes are checked, anyone who has write access can merge the PR.

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (ffc8b71) 14.52% compared to head (eaa9968) 14.52%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3776   +/-   ##
=======================================
  Coverage   14.52%   14.52%           
=======================================
  Files        1442     1442           
  Lines      101463   101463           
  Branches    30156    30156           
=======================================
  Hits        14741    14741           
  Misses      70584    70584           
  Partials    16138    16138           

Flag	Coverage Δ		*Carryforward flag
differential	11.09% <ø> (?)
total	14.52% <ø> (ø)		Carriedforward from ffc8b71

*This pull request uses carry forward flags. Click here to find out more.

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[takayuki5168]

I added code owners. cc @TakaHoribe @rej55 @kyoichi-sugahara