Fixed charset setting for checkstyle.

Checkstyle should not rely on the file.encoding property but always use
UTF-8.

Author: cboehme

quality_assurance/checkstyle_rules.xml

@@ -10,6 +10,7 @@
 -->
 <module name="Checker">
   <property name="severity" value="warning"/>
+  <property name="charset" value="UTF-8"/>
   <module name="TreeWalker">
     <module name="FileContentsHolder"/>
     <module name="ClassTypeParameterName"/>