[plasmalnf] CMake correction

libplasma is not required, if not found just skip the module, like in kf5

src/modules/plasmalnf/CMakeLists.txt

@@ -13,7 +13,7 @@ if(WITH_QT6)
     set(_plasma_libraries "Plasma::Plasma")
     set(_plasma_name "Plasma")
     find_package(${kfname} ${KF_VERSION} QUIET COMPONENTS Config Package)
-    find_package(Plasma ${PLASMA_VERSION} REQUIRED)
+    find_package(Plasma ${PLASMA_VERSION})
 else()
     set(_plasma_libraries "${kfname}::Plasma")
     set(_plasma_name "KF5Plasma")