Fix jdbc-jc to work with tomcat gradle plugin

It is necessary to ensure that src/main/webapp exists to ensure the
application starts with the Tomcat Gradle Plugin.

This commit adds a Manifest file to src/main/webapp/META-INF to ensure
that git contains the otherwise empty directory.

samples/jdbc-jc/src/main/webapp/META-INF/MANIFEST.MF

@@ -0,0 +1,2 @@
+Manifest-Version: 1.0
+