Allow Qt to search for Qt plugins (e.g. sqldrivers) in our plugin directory

Without this it is necessary to explicitly provide QT_PLUGIN_PATH environment
variable for Qt plugins we build (e.g. Oracle DB sql driver)

Author: wonder-sk

src/core/qgsapplication.cpp

@@ -178,6 +178,8 @@ void QgsApplication::init( QString customConfigPath )
   }
   ABISYM( mSystemEnvVars ) = systemEnvVarMap;
 
+  // allow Qt to search for Qt plugins (e.g. sqldrivers) in our plugin directory
+  QCoreApplication::addLibraryPath( pluginPath() );
 }
 
 QgsApplication::~QgsApplication()