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: https://github.com/apache/maven-compiler-plugin/blob/d79caa02f7b4b7efca14d178a1cc115ebf50e724/src/main/java/org/apache/maven/plugin/compiler/CompilerMojo.java#L394-L395

This was introduced in https://github.com/apache/maven-compiler-plugin/pull/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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