gitignore: add coverage file

[sebix]

add the file name coverage.xml to the gitignore patterns
the file is a result of a test coverage analysis, not to be committed to
the repository

[codecov-commenter]

Codecov Report

Merging #2182 (b1ab3de) into develop (ce60fd4) will decrease coverage by 0.40%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #2182      +/-   ##
===========================================
- Coverage    76.34%   75.93%   -0.41%     
===========================================
  Files          441      441              
  Lines        23649    23318     -331     
  Branches      3739     3434     -305     
===========================================
- Hits         18055    17707     -348     
- Misses        4857     4872      +15     
- Partials       737      739       +2     

Impacted Files	Coverage Δ
intelmq/bots/experts/mcafee/expert_mar.py	35.55% <0.00%> (-8.89%)	⬇️
intelmq/bots/experts/cymru_whois/_lib.py	17.02% <0.00%> (-5.76%)	⬇️
intelmq/bots/experts/modify/expert.py	92.00% <0.00%> (-5.34%)	⬇️
intelmq/bots/parsers/dshield/parser_domain.py	95.45% <0.00%> (-4.55%)	⬇️
intelmq/tests/lib/test_exceptions.py	95.65% <0.00%> (-4.35%)	⬇️
intelmq/bots/experts/abusix/_lib.py	36.84% <0.00%> (-3.16%)	⬇️
intelmq/tests/bots/collectors/rt/test_collector.py	69.44% <0.00%> (-3.06%)	⬇️
intelmq/bots/parsers/dshield/parser_block.py	87.50% <0.00%> (-2.50%)	⬇️
intelmq/bots/collectors/opendxl/collector.py	31.14% <0.00%> (-2.19%)	⬇️
intelmq/tests/bots/experts/wait/test_expert.py	44.00% <0.00%> (-2.16%)	⬇️
... and 259 more

[waldbauer-certat]

LGTM