Build/Test Tools: Exclude gutenberg directory when running PHPCS.

The relevant files will be scanned within `src` (provided the build script has been run).

See #64394.

git-svn-id: https://develop.svn.wordpress.org/trunk@61726 602fd350-edb4-49c9-b593-d223f7449a82

Author: desrosj

phpcs.xml.dist

@@ -42,6 +42,9 @@
 	<!-- Exclude the build folder in the current directory. -->
 	<exclude-pattern type="relative">^build/*</exclude-pattern>
 
+	<!-- Exclude the local copy of the Gutenberg repository. -->
+	<exclude-pattern>/gutenberg/*</exclude-pattern>
+
 	<!-- Directories and third party library exclusions. -->
 	<exclude-pattern>/node_modules/*</exclude-pattern>
 	<exclude-pattern>/vendor/*</exclude-pattern>