When cross-compiling qtlanguageserver module should be skipped

When cross-compiling, whole qtlanguageserver module should be skipped completely (if it has been checked out) as its not needed at all.

– Searching for tool 'Qt6::qmlls' in package Qt6QmlTools.
CMake Error at qtbase/cmake/QtToolHelpers.cmake:170 (message):
 The tool "Qt6::qmlls" was not found in the Qt6QmlTools package.  Package
 found: 1
Call Stack (most recent call first):
 qtdeclarative/tools/qmlls/CMakeLists.txt:6 (qt_internal_add_tool)