Add a missing include of <stdio.h>

[stolyaroleh]

Fixes the following error when building with Clang 9 and libc++ on Linux:

clang++  -O2 -std=c++14  -o build/src/Allocator.o -c src/Allocator.cpp
src/Allocator.cpp:45:13: error: use of undeclared identifier 'printf'
            printf("ERROR: failed to allocate %zu bytes via mmap\n", count);
            ^
1 error generated.

[stolyaroleh]

Relevant: #12 (comment)