Always link to libtinyxml2

In many cases it is not needed, but I am too lazy to write
a proper test for it right now.

src/CMakeLists.txt

@@ -10,5 +10,6 @@ foreach(_program ${programs})
   add_executable(${_program} ${_program}.cc)
   target_link_dune_default_libraries(${_program})
   add_dune_pythonlibs_flags(${_program})
+  target_link_libraries(${_program} tinyxml2)
 #  target_compile_options(${_program} PRIVATE "-fpermissive")
 endforeach()