Make Romain a code owner for symex, per Peter Schrammel's recommendation

[smowton]

No description provided.

[allredj]

⚠️
This PR failed Diffblue compatibility checks (cbmc commit: c7b1bc7).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/129245447
Status will be re-evaluated on next push.
Common spurious failures include: the cbmc commit has disappeared in the mean time (e.g. in a force-push); the author is not in the list of contributors (e.g. first-time contributors); compatibility was already broken by an earlier merge.

[codecov-io]

Codecov Report

Merging #5130 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #5130   +/-   ##
========================================
  Coverage    66.97%   66.97%           
========================================
  Files         1146     1146           
  Lines        93746    93746           
========================================
  Hits         62787    62787           
  Misses       30959    30959

Flag	Coverage Δ
#cproversmt2	42.66% <ø> (ø)	⬆️
#regression	63.47% <ø> (ø)	⬆️
#unit	31.9% <ø> (ø)	⬆️

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