WIN32: Fix pGetSystemWindowsDirectory found initialized during FcConfigParseAndLoadFromMemory

Trying to early loading a custom fonts.xml created on the fly results in the pGetSystemWindowsDirectory function pointer being found initialized. Normally the initialization is performed in the default configuration loading. The commit factorizes the lazy initialization of both WIN32 getters (together with pSHGetFolderPathA) in a function and call it when actually needed.