My advapi32.dll, on Windows 10 22H2, does not export this function. Just checked that.
Might be one of those cases where the actual DLL entry point is called, e.g., "SystemFunctionXYZ" and you have to use LoadLibrary() + GetProcAddress() trickery to call it. But then you wouldn't get a "The procedure entry point could not be located" error at startup!
However, a full-text search for "GetMangledSiteSid" on latest MSYS2/MinGW does not find anything either. So, apparently, there is no header file that declares this function. Makes it even more dubious how the OP actually calls this function...
So, its an old undocumented function that has long been removed.
This explains why there is no sign of GetMangledSiteSid() in current Mingw-w64. I don't know how the OP managed to get a dependency on this function, because there seems to be no header file (or import library) that contains the function in question 🤔
Two possibilities come to my mind:
• OP is using a very old MinGW version from the early 2000's
• It's an indirect dependency that was induced by some 3rd-party DLL, one that was made to run on Windows 2000