Install service menus in ${KDE_INSTALL_KSERVICES5DIR}/ServiceMenus

Task: https://phabricator.kde.org/T14543

ksycoca already indexes the subdirectories, consequently this change
won't cause any incompatibilities.

src/CMakeLists.txt

@@ -384,5 +384,5 @@ install(
         data/kleopatra_signencryptfolders.desktop
         data/kleopatra_decryptverifyfiles.desktop
         data/kleopatra_decryptverifyfolders.desktop
-  DESTINATION ${KDE_INSTALL_KSERVICES5DIR}
+  DESTINATION ${KDE_INSTALL_KSERVICES5DIR}/ServiceMenus
 )