Set explicit line endings for all *.java files

Commit 5b250e2 introduced the Spotless plugin to our build process.
From this commit on I was not able to run `gradlew build` on my Windows
machine. Spotless failed because it expected all *.java files to have
the CRLF line endings (system default), but due to Git's
core.autocrlf=false these files had LF line endings.

Spotless uses by default the Git settings to determine the expected line
endings for the checked files. Instead of configure Spotless to use an
explicit line ending, their recommendation is to set them via the
.gitattributes file. [1]

[1] https://github.com/diffplug/spotless/tree/main/plugin-gradle#line-endings-and-encodings-invisible-stuff

Signed-off-by: Roland Weisleder <roland.weisleder@googlemail.com>

.gitattributes

@@ -0,0 +1 @@
+*.java text eol=lf