Fix build on gcc8

tests/slic3rutils/CMakeLists.txt

@@ -3,7 +3,7 @@ add_executable(${_TEST_NAME}_tests
     ${_TEST_NAME}_tests_main.cpp
     )
 
-target_link_libraries(${_TEST_NAME}_tests test_common libslic3r_gui)
+target_link_libraries(${_TEST_NAME}_tests test_common libslic3r_gui libslic3r)
 set_property(TARGET ${_TEST_NAME}_tests PROPERTY FOLDER "tests")
 
 # catch_discover_tests(${_TEST_NAME}_tests TEST_PREFIX "${_TEST_NAME}: ")