headers: Fix a use of WINAPI_FAMILY_PARTITION in fileapi.h

WINAPI_FAMILY_PARTITION is only supposed to be used with
WINAPI_PARTITION_* constants, not with WINAPI_FAMILY_*_APP constants.

However, all the functions in this block actually are available
for WINAPI_PARTITION_APP already since windows 8.0, so change to use
that instead of WINAPI_PARTITION_DESKTOP.

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