Ignore Gradle plugin build output

Author: philwebb

src/checkstyle/checkstyle-suppressions.xml

@@ -3,6 +3,7 @@
 	"-//Checkstyle//DTD SuppressionFilter Configuration 1.2//EN"
 	"https://checkstyle.org/dtds/suppressions_1_2.dtd">
 <suppressions>
+	<suppress files="[\\/]spring-boot-gradle-plugin[\\/]build" checks=".*" />
 	<suppress files="[\\/]\.flattened-pom\.xml" checks=".*" />
 	<suppress files="[\\/]transaction-logs[\\/]" checks=".*" />
 	<suppress files="[\\/]target[\\/]" checks=".*" />