Made the osg::ApplicationUsage forward declared.

Added include of ostream to ArgumentParser header.

src/osg/ArgumentParser.cpp

@@ -1,7 +1,15 @@
 #include <osg/ArgumentParser>
+#include <osg/ApplicationUsage>
 
 using namespace osg;
 
+ArgumentParser::ArgumentParser(int* argc,char **argv):
+    _argc(argc),
+    _argv(argv),
+    _usage(ApplicationUsage::instance())
+{
+}
+
 std::string ArgumentParser::getProgramName() const
 {
     if (_argc>0) return std::string(_argv[0]);

src/osg/DisplaySettings.cpp

@@ -11,6 +11,7 @@
  * OpenSceneGraph Public License for more details.
 */
 #include <osg/DisplaySettings>
+#include <osg/ArgumentParser>
 #include <osg/ApplicationUsage>
 #include <osg/ref_ptr>