fixed `-Wmissing-prototypes` (and subsequent) compiler warnings #1601

Job	Run time
code formatting check	44s
cppcheck	10m 20s
min features with gcc	2m 40s
min features with g++	2m 15s
min features with clang	2m 21s
max features with gcc	4m 1s
max features with g++	3m 3s
max features with clang	4m 14s
max features with gcc and DEBUG	3m 0s
max features with gcc for 32-bit arch (legacy OS)	1m 54s
max features with g++ for 32-bit arch (legacy OS)	2m 3s
max features with clang for 32-bit arch (legacy OS)	2m 42s
	39m 17s

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