build.gradle: fix checkstyle plugin failure

The `checkstyle` plugin of Gradle fails in JDK11 and gives the error
below:

    Unable to create Root Module: config ...

The main reason is that, in JDK11, the `user.dir` cannot be reset by
Gradle [1]. So, checkstyle plugin is unable to locate the suppressions
file correctly.

Let's add `config_loc` variable suggested by Gradle to solve the
problem [1].

[1] gradle/gradle#8286

config/checkstyle/checkstyle.xml

@@ -26,7 +26,7 @@
   </module>
 
   <module name="SuppressionFilter">
-    <property name="file" value="config/checkstyle/suppressions.xml"/>
+    <property name="file" value="${config_loc}/suppressions.xml"/>
   </module>