fixed `-Wmissing-prototypes` (and subsequent) compiler warnings #1598

Job	Run time
cppcheck	13m 24s
code formatting check	34s
min features with gcc	3m 12s
min features with g++	2m 43s
min features with clang	3m 9s
max features with gcc	3m 20s
max features with g++	2m 48s
max features with clang	3m 37s
max features with gcc and DEBUG	2m 35s
max features with gcc for 32-bit arch (legacy OS)	2m 18s
max features with g++ for 32-bit arch (legacy OS)	2m 26s
max features with clang for 32-bit arch (legacy OS)	2m 34s
	42m 40s

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