Do not update mimetypes when crosscompiling

When crosscompiling update_xdg_mimetypes would create
empty mime data for the target system.

src/mimetypes/CMakeLists.txt

@@ -9,6 +9,6 @@ set_package_properties(SharedMimeInfo PROPERTIES
                       )
 
 # update XDG mime-types if shared mime info is around
-if(SharedMimeInfo_FOUND)
+if(SharedMimeInfo_FOUND AND NOT CMAKE_CROSSCOMPILING)
     update_xdg_mimetypes(${XDG_MIME_INSTALL_DIR})
 endif()