Actually make the Qt5Positioning dependency optional

The surrounding code handles this just fine, just the checking code didn't.

src/lib/marble/declarative/CMakeLists.txt

 PROJECT( MarbleDeclarativePlugin )
 
-find_package(Qt5 ${REQUIRED_QT_VERSION} COMPONENTS Positioning)
+find_package(Qt5 ${REQUIRED_QT_VERSION} OPTIONAL_COMPONENTS Positioning)
 if(Qt5Positioning_FOUND)
     add_definitions(-DHAVE_QT5_POSITIONING)
 endif(Qt5Positioning_FOUND)

src/plugins/positionprovider/CMakeLists.txt

@@ -9,7 +9,7 @@ IF(LIBGPS_FOUND)
 ENDIF(LIBGPS_FOUND)
 
 find_package(Qt5 ${REQUIRED_QT_VERSION}
-    COMPONENTS
+    OPTIONAL_COMPONENTS
         Positioning
 )
 marble_set_package_properties( Qt5Positioning PROPERTIES DESCRIPTION "a collection of APIs and frameworks" )