Configure Checkstyle line separator so it does not use system default

In its default configuration, Checkstyle would fail on Windows as it
would expect CRLF, the system default line separator.

Closes gh-4666

spring-boot-parent/src/checkstyle/checkstyle.xml

@@ -7,7 +7,9 @@
 		<property name="headerFile" value="${checkstyle.header.file}" />
 		<property name="fileExtensions" value="java" />
 	</module>
-	<module name="NewlineAtEndOfFile" />
+	<module name="NewlineAtEndOfFile">
+		<property name="lineSeparator" value="lf"/>
+	</module>
 
 	<!-- TreeWalker Checks -->
 	<module name="TreeWalker">