libavutil: Make avpriv_open a library-internal function on msvcrt

libavutil: Make avpriv_open a library-internal function on msvcrt

Add one copy of the function into each of the libraries, similarly
to what we do for log2_tab. When using static libs, only one
copy of the file_open.o object file gets included, while when
using shared libraries, each of them get a copy of its own.

This fixes DLL builds with a statically linked C runtime, where
each DLL effectively has got its own instance of the C runtime,
where file descriptors can't be shared across runtimes.

On systems not using msvcrt, the function is not duplicated.

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