libavformat: Only use MoveFileExA when targeting the desktop API subset

The MoveFileExA is available in the headers regardless which API
subset is targeted, but it is missing in the Windows Phone link
libraries. When targeting Windows Store apps, the function is
available both in the headers and in the link libraries, and thus
there is no indication for the build system that this function
should be avoided - such an indication is only given by the
Windows App Certification Kit, which forbids using the MoveFileExA
function.

Therefore check the WINAPI_FAMILY defines instead, to figure out
which API subset is targeted.

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