fixed `-Wmissing-prototypes` (and subsequent) compiler warnings #1600

Job	Run time
code formatting check	27s
cppcheck	11m 5s
min features with gcc	3m 20s
min features with g++	2m 45s
min features with clang	3m 35s
max features with gcc	3m 21s
max features with g++	2m 49s
max features with clang	4m 56s
max features with gcc and DEBUG	3m 48s
max features with gcc for 32-bit arch (legacy OS)	2m 39s
max features with g++ for 32-bit arch (legacy OS)	2m 2s
max features with clang for 32-bit arch (legacy OS)	2m 10s
	42m 57s

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