Export CTK_INSTALL_QTPLUGIN_DIR for external use

CTKConfig.cmake.in

@@ -165,6 +165,7 @@ SET(CTK_CMAKE_UTILITIES_DIR "@CTK_CMAKE_UTILITIES_DIR_CONFIG@")
 SET(CTK_INSTALL_BIN_DIR "@CTK_INSTALL_BIN_DIR@")
 SET(CTK_INSTALL_INCLUDE_DIR "@CTK_INSTALL_INCLUDE_DIR@")
 SET(CTK_INSTALL_LIB_DIR "@CTK_INSTALL_LIB_DIR@")
+SET(CTK_INSTALL_QTPLUGIN_DIR "@CTK_INSTALL_QTPLUGIN_DIR@")
 #SET(CTK_INSTALL_PACKAGE_DIR "@CTK_INSTALL_PACKAGE_DIR@")
 
 # A CTK install tree always provides one build configuration. A CTK