cmake fixes for external OndselSolver building (#18379)

* Assembly: fix building with external OndselSolver

This commit pairs with f35b075e271e9cb3451f3439f7dc0274841a0838.
It solves two issues:

- remove hardcoded includes of 3rdparty files
- add missing include_directories()

* Assembly: add check for external OndselSolver includes

tests/src/Mod/Assembly/CMakeLists.txt

@@ -5,6 +5,11 @@ target_include_directories(Assembly_tests_run PUBLIC
     ${Python3_INCLUDE_DIRS}
     ${XercesC_INCLUDE_DIRS}
 )
+if (NOT FREECAD_USE_EXTERNAL_ONDSELSOLVER)
+	target_include_directories(Assembly_tests_run PUBLIC
+		${CMAKE_SOURCE_DIR}/src/3rdParty/OndselSolver
+	)
+endif ()
 target_link_directories(Assembly_tests_run PUBLIC ${OCC_LIBRARY_DIR})
 
 target_link_libraries(Assembly_tests_run