headers: Deduplicate *wprintf/*wscanf functions across UCRT/msvcrt builds

All these functions have same declarations in #ifdef _UCRT and #else
blocks. The only difference is that in non-UCRT block functions vfwscanf,
vswscanf and vwscanf are declared without __cdecl. As all those functions
are with __cdecl calling convenion, they should be declared with __cdecl.

Signed-off-by: Martin Storsjö <martin@martin.st>