headers: Fix the declaration of _vscprintf

Previously, the ifdefs were misplaced, leading to this function not
being declared at all if building with __USE_MINGW_ANSI_STDIO=1.

If targeting ucrtbase.dll, we always need to rely on the
__stdio_common_vsprintf implementation, since _vscprintf doesn't
exist in ucrtbase.dll, regardless of the state of the
__USE_MINGW_ANSI_STDIO define.

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