Fix CompilerMojo#getSourceInclusionScanner(String) illogical addAll

[JackPGreen]

Sonar identified that usage of addAll in CompilerMojo#getSourceInclusionScanner(String) was illogical - i.e. adding excludesIncr into itself:

maven-compiler-plugin/src/main/java/org/apache/maven/plugin/compiler/CompilerMojo.java

Lines 394 to 395 in d79caa0

	Set<String> excludesIncr = new HashSet<>(excludes);
	excludesIncr.addAll(excludesIncr);

This was introduced in #141, and the same snippet is used in multiple places - only one has this defect, which I suspect is a copy-paste error.

Changes:

• updated to be consistent with the other examples
• removed the this. qualifier, as the existing incrementalExcludes access in the same method doesn't use it to be consistent

I am not aware of any defect arising from this issue.

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[JackPGreen]

@gnodet thanks for the approval - are you able to merge, please?