idlc: clear include file set in Idlc::reset():

Resetting the set between files reduces the size of the generated
offapi.d from 41M to 4.5M.

Change-Id: I221e6dfb75cbadb5d970f18eccfc85ffdb83ce6c

idlc/source/idlc.cxx

@@ -270,6 +270,8 @@ void Idlc::reset()
     // push the root node on the stack
     m_pScopes->push(m_pRoot);
     initializePredefinedTypes(m_pRoot);
+
+    m_includes.clear();
 }
 
 sal_Bool Idlc::isDocValid()