[core] Fix some warnings when compiling with mingw

[yomnes0]

fixes #2862
Removed duplicate declaration of SRT_API
Moved around variables to avoid set but not used warnings

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 48.81%. Comparing base (4c443d6) to head (55c028c).
Report is 18 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #2868       +/-   ##
===========================================
- Coverage   67.26%   48.81%   -18.46%     
===========================================
  Files         103      141       +38     
  Lines       20460    23356     +2896     
===========================================
- Hits        13763    11401     -2362     
- Misses       6697    11955     +5258     

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[maxsharabayko]

I believe the definition of the SRT_API must stay in srt.h.

This was also noted by @ethouris in his comment

The definition of SRT_API should still stay in this file, but then this symbol shall not be used anywhere else in the SRT code than this exactly file.

[Andarwinux]

This causes ffmpeg can't find srt_compat.h when libsrt is enabled.

x86_64-w64-mingw32/include/srt/srt.h:39:10: fatal error: 'srt_compat.h' file not found
   39 | #include "srt_compat.h"
      |          ^~~~~~~~~~~~~~
1 error generated.
ERROR: srt >= 1.3.0 not found using pkg-config

[yomnes0]

This include should not be there, it will be fixed soon