mfenc: Fix building with clang in MSVC mode

mfenc: Fix building with clang in MSVC mode

Just including icodecapi.h + codecapi.h apparently misses some bits,
that MSVC doesn't complain about, but Clang does. Thus add a missing
header to the custom hacky include path (required for keeping
ICodecAPI visible when building in UWP mode, as it is intended to).

The issue in Windows SDK headers about ICodecAPI availability in
UWP/app mode has been reported upstream at
https://developercommunity.visualstudio.com/content/problem/1037125/icodecapi-not-visible-when-compiling-in-uwp-mode.html,
and later also filed in Feedback Hub (which can't be linked to
externally).

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