avfilter: Make avpriv_open a library-internal function on msvcrt

This applies the same change for libavfilter as commit
e743e7a did for the other libraries.

Original commit message:

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.

Based-on-patch-by: Martin Storsjö <martin@martin.st>
Signed-off-by: softworkz <softworkz@hotmail.com>

libavfilter/Makefile

@@ -23,6 +23,7 @@ OBJS = allfilters.o                                                     \
        version.o                                                        \
        video.o                                                          \
 
+OBJS-$(HAVE_LIBC_MSVCRT)                     += file_open.o
 OBJS-$(HAVE_THREADS)                         += pthread.o
 
 # subsystems

libavfilter/file_open.c

@@ -0,0 +1 @@
+#include "libavutil/file_open.c"