🐛 fix missing mcx gate support in OpenQASM3 parser

[burgholzer]

Description

This fixes a small regression in the new QASM parser where plain mcx statements were no longer accepted.

Checklist:

• The pull request only contains commits that are related to it.
• I have added appropriate tests and documentation.
• I have made sure that all CI jobs on GitHub pass.
• The pull request introduces no new warnings and follows the project's style guidelines.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (ba9ba16) 91.3% compared to head (cb0daf3) 91.3%.

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #526   +/-   ##
=====================================
  Coverage   91.3%   91.3%           
=====================================
  Files        128     128           
  Lines      13794   13795    +1     
  Branches    2149    2149           
=====================================
+ Hits       12601   12602    +1     
  Misses      1193    1193           

Flag	Coverage Δ
cpp	91.0% <100.0%> (+<0.1%)	⬆️
python	99.7% <ø> (ø)

Files	Coverage Δ
src/parsers/QASM3Parser.cpp	90.5% <100.0%> (+<0.1%)	⬆️