Fix NoClassDefFoundError when execute dhcp-set-static-mapping

The exception occurs because dhcp maven-bundle-plugin does not
properly export StringCompleter class which is contained under
org.apache.karaf.shell.console.completer package.

Change-Id: I9120e1b5b874de94d759e9fee41dcc660f24d793
diff --git a/apps/dhcp/app/pom.xml b/apps/dhcp/app/pom.xml
index 42c31b7..ad28cb3 100644
--- a/apps/dhcp/app/pom.xml
+++ b/apps/dhcp/app/pom.xml
@@ -148,6 +148,7 @@
                             com.fasterxml.jackson.core,
                             org.apache.karaf.shell.commands,
                             org.apache.karaf.shell.console,
+                            org.apache.karaf.shell.console.completer,
                             com.google.common.*,
                             org.onlab.packet.*,
                             org.onlab.rest.*,