2 Compiler warnings on Msys2, Mingw64, GCC

I'm trying to minimize warnings in our builds so just reporting 2 which are coming from mimalloc sources:

```
D:/a/openscad/openscad/submodules/mimalloc/src/os.c:376:5: warning: missing initializer for field 'AllocationProtect' of 'MEMORY_BASIC_INFORMATION' [-Wmissing-field-initializers]
376 | MEMORY_BASIC_INFORMATION info = { 0, 0 };
| ^~~~~~~~~~~~~~~~~~~~~~~~
In file included from D:/a/_temp/msys64/mingw64/include/minwindef.h:163,
from D:/a/_temp/msys64/mingw64/include/windef.h:9,
from D:/a/_temp/msys64/mingw64/include/windows.h:69,
from D:/a/openscad/openscad/submodules/mimalloc/include/mimalloc-atomic.h:286,
from D:/a/openscad/openscad/submodules/mimalloc/include/mimalloc-types.h:13,
from D:/a/openscad/openscad/submodules/mimalloc/include/mimalloc-internal.h:11,
from D:/a/openscad/openscad/submodules/mimalloc/src/os.c:20:
D:/a/_temp/msys64/mingw64/include/winnt.h:5395:13: note: 'AllocationProtect' declared here
5395 | DWORD AllocationProtect;
| ^~~~~~~~~~~~~~~~~
```

```
D:/a/openscad/openscad/submodules/mimalloc/src/os.c:378:88: warning: comparison of integer expressions of different signedness: 'long long int' and 'long long unsigned int' [-Wsign-compare]
378 | if (info.AllocationBase < addr && ((uint8_t*)addr - (uint8_t*)info.AllocationBase) < MI_SEGMENT_SIZE) {
| ^
```

mimalloc 1.7.6
Built on Windows using MSYS2 / mingw-w64 / gcc

Start in submodules/mimalloc/src/os.c around lines 376-378, where the MSYS2/MinGW64 GCC warnings are reported. Reproduce the build with mimalloc 1.7.6 using Windows MSYS2/MinGW64, then verify that both the missing-initializer and signedness warnings are resolved without introducing new warnings.

Written by the indexing model from the issue text.