Fix swap_cols! for Matrix{T}

[fingolfin]

This is also in PR #2051 (where we discovered the issue) but as that seems stalled let's apply this fix right now.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 88.37%. Comparing base (c5edd01) to head (6c45392).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/julia/Matrix.jl	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2065   +/-   ##
=======================================
  Coverage   88.37%   88.37%           
=======================================
  Files         125      125           
  Lines       31585    31585           
=======================================
  Hits        27912    27912           
  Misses       3673     3673           

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[joschmitt]

It's not great that this is not tested, but #2050 should take care of that at some point...