Fix marshalling of StringBuilder errors

[kayoub5]

Fixes #532
@trivalik Please test and let me know.

[codecov]

Codecov Report

Attention: Patch coverage is 79.31034% with 6 lines in your changes missing coverage. Please review.

Project coverage is 81.25%. Comparing base (2fff830) to head (3b1b506).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
SharpPcap/LibPcap/PcapInterface.cs	40.00%	3 Missing ⚠️
SharpPcap/LibPcap/LibPcapLiveDevice.cs	66.66%	2 Missing ⚠️
SharpPcap/LibPcap/CaptureFileReaderDevice.cs	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #533      +/-   ##
==========================================
- Coverage   81.48%   81.25%   -0.24%     
==========================================
  Files          51       51              
  Lines        2825     2811      -14     
  Branches      310      307       -3     
==========================================
- Hits         2302     2284      -18     
- Misses        421      430       +9     
+ Partials      102       97       -5     

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[trivalik]

I can't undererstand why not remove the not used using-directives, but ok, for final outcome does it not matter.