Fix build failure when Qt Location is not available

Adds parenthesis to fix the precedence in the existing test and adds
another condition that makes the presence of Qt Location actually
optional like in the top level CMakeLists.txt

plugins/feature/CMakeLists.txt

@@ -13,7 +13,7 @@ else()
 endif()
 
 # WebEngine on Qt5, WebEngineCore on Qt6
-if(ENABLE_FEATURE_SKYMAP AND Qt${QT_DEFAULT_MAJOR_VERSION}WebEngine_FOUND OR Qt${QT_DEFAULT_MAJOR_VERSION}WebEngineCore_FOUND)
+if(ENABLE_FEATURE_SKYMAP AND (Qt${QT_DEFAULT_MAJOR_VERSION}WebEngine_FOUND OR Qt${QT_DEFAULT_MAJOR_VERSION}WebEngineCore_FOUND) AND Qt${QT_DEFAULT_MAJOR_VERSION}Location_FOUND)
     add_subdirectory(skymap)
 endif()