smack-java7: Add missing smack-xmlparser-stax dependency

Since 2f667f95a ("gradle: Remove archives configuration") the

:smack-repl:printXmppNioTcpConnectionStateGraph

JavaExec task failed with

> Task :smack-repl:printXmppNioTcpConnectionStateGraph FAILED
Exception in thread "main" java.lang.ExceptionInInitializerError
        at org.jivesoftware.smack.SmackConfiguration.getVersion(SmackConfiguration.java:108)
        at org.jivesoftware.smack.AbstractXMPPConnection.<clinit>(AbstractXMPPConnection.java:187)
        at org.igniterealtime.smack.smackrepl.StateGraph.main(StateGraph.java:37)
Caused by: java.lang.IllegalStateException: Could not parse Smack configuration file
        at org.jivesoftware.smack.SmackInitialization.<clinit>(SmackInitialization.java:100)
        ... 3 more
Caused by: java.lang.IllegalStateException: Could not load a XmlPullParserFactory via Service Provider Interface (SPI)
        at org.jivesoftware.smack.xml.SmackXmlParser.getXmlPullParserFactory(SmackXmlParser.java:34)
        at org.jivesoftware.smack.xml.SmackXmlParser.newXmlParser(SmackXmlParser.java:53)
        at org.jivesoftware.smack.util.PacketParserUtils.getParserFor(PacketParserUtils.java:76)
        at org.jivesoftware.smack.SmackInitialization.processConfigFile(SmackInitialization.java:144)
        at org.jivesoftware.smack.SmackInitialization.processConfigFile(SmackInitialization.java:139)
        at org.jivesoftware.smack.SmackInitialization.<clinit>(SmackInitialization.java:97)
        ... 3 more

because no XmlPullParser was registered via SPI. The 'archives'
configuration which was removed with 2f667f95a ("gradle: Remove
archives configuration"), previously pulled in the parser.

Just like smack-android delcares a dependency on smack-xmlparser-xpp3,
smack-java7 should declare a dependency on smack-xmlparser-stax.

smack-java7/build.gradle

@@ -8,6 +8,7 @@ dependencies {
 	compile project(":smack-core")
 	compile project(":smack-resolver-javax")
 	compile project(":smack-sasl-javax")
+	implementation project(":smack-xmlparser-stax")
 }
 
 javadoc {