protect freqresp against loading GenericLinearAlgebra

[baggepinnen]

GLA returns it's own Hessenberg struct that does not have Q, so loading GLA might break freqresp due to GLA doing type piracy :/

• HessenbergFactorization does not have property Q JuliaLinearAlgebra/GenericLinearAlgebra.jl#84

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Plots were compared to references. 9/11 images have changed, see differences below.
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Difference	Reference Image	New Image
✔️ 0.003
✔️ 0.003
✔️ 0.001
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✔️ 0.0
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[codecov]

Codecov Report

Merging #637 (f5ecd73) into master (1d81af0) will not change coverage.
The diff coverage is 50.00%.

@@           Coverage Diff           @@
##           master     #637   +/-   ##
=======================================
  Coverage   86.73%   86.73%           
=======================================
  Files          31       31           
  Lines        3248     3248           
=======================================
  Hits         2817     2817           
  Misses        431      431           

Impacted Files	Coverage Δ
src/freqresp.jl	96.24% <50.00%> (ø)

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[JuliaControlBot]

This is an automated message.
Plots were compared to references. 9/11 images have changed, see differences below.
After pulling this PR, please update the reference images by creating a PR to ControlExamplePlots.jl here.

Difference	Reference Image	New Image
✔️ 0.003
✔️ 0.003
✔️ 0.001
✔️ 0.001
✔️ 0.0
✔️ 0.0
❌ 0.042
✔️ 0.006
✔️ 0.011