avutil/wchar_filename: re-introduce explicit cast of void* to char*

Fixes compile error on windows with decklink:

In file included from ./libavformat/os_support.h:175,
                 from ./libavformat/internal.h:30,
                 from libavdevice/decklink_common.cpp:25:
./libavutil/wchar_filename.h: In function 'int wchartocp(unsigned int, const wchar_t*, char**)':
./libavutil/wchar_filename.h:59:32: error: invalid conversion from 'void*' to 'char*' [-fpermissive]
   59 |     *filename = av_malloc_array(num_chars, sizeof **filename);
      |                 ~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      |                                |
      |                                void*

Regression since e9e8bea2e7.

Fixes ticket #11103.

Signed-off-by: Marton Balint <cus@passwd.hu>
(cherry picked from commit 9b0128aa76)

libavutil/wchar_filename.h

@@ -57,7 +57,7 @@ static inline int wchartocp(unsigned int code_page, const wchar_t *filename_w,
         errno = EINVAL;
         return -1;
     }
-    *filename = av_malloc_array(num_chars, sizeof **filename);
+    *filename = (char *)av_malloc_array(num_chars, sizeof **filename);
     if (!*filename) {
         errno = ENOMEM;
         return -1;