Fix: Add missing headers in order to import definitions. #565

Job	Run time
t8code_w_shipped_submodules_tests / t8code_cmake_tests	5m 16s
preparation (OFF) / cmake_preparation	1m 13s
preparation (ON) / cmake_preparation	1m 12s
sc_p4est_tests (OFF) / sc_p4est_cmake_tests	3m 54s
t8code_api_tests (ON, Release) / t8code_cmake_tests	3m 44s
sc_p4est_tests (ON) / sc_p4est_cmake_tests	3m 38s
t8code_api_tests (ON, Debug) / t8code_cmake_tests	4m 55s
t8code_linkage_tests (OFF, Debug) / t8code_cmake_tests	8m 34s
t8code_linkage_tests (OFF, Release) / t8code_cmake_tests	7m 23s
t8code_linkage_tests (ON, Debug) / t8code_cmake_tests	13m 8s
t8code_linkage_tests (ON, Release) / t8code_cmake_tests	9m 35s
t8code_tests (OFF, Debug) / t8code_cmake_tests	3m 17s
t8code_tests (OFF, Release) / t8code_cmake_tests	2m 47s
t8code_tests (ON, Debug) / t8code_cmake_tests	4m 58s
t8code_tests (ON, Release) / t8code_cmake_tests	3m 44s
	1h 17m 18s

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