Fix compilation on non recent windows SDKs (#1399)

Fix compilation on non recent windows SDKs (#1399)

In commit f8bb9d8 the header stringapiset.h was included, very likely because the user who made the change saw that the funciton WideCharToMultiByte is "declared" in there.
That header tho is a recent addition to the windows headers added in the last years in an attempt from microsoft to split the windows.h header in multiple files, so the inclusion fails when the library is not built with the latest visual studio using the latest windows 10 sdk.
That inclusion can be safely removed as in any case the function WideCharToMultiByte was already included by the windows.h header that is included few lines below.