avisynth: adapt 239d02ef to dlsym

This commit was merged in a couple years ago as a no-op because we
had already switched from GetProcAddress to dlsym some time before
that.  However, not applying the actual cast causes warnings about
FARPROC and when attempting to build FFmpeg in MSVC with AviSynth-GCC
32-bit compatibility, those FARPROC warnings turn into FARPROC errors.

libavformat/avisynth.c

@@ -123,7 +123,8 @@ static av_cold int avisynth_load_library(void)
         return AVERROR_UNKNOWN;
 
 #define LOAD_AVS_FUNC(name, continue_on_fail)                          \
-        avs_library.name = dlsym(avs_library.library, #name);          \
+        avs_library.name = (name ## _func)                             \
+                           dlsym(avs_library.library, #name);          \
         if (!continue_on_fail && !avs_library.name)                    \
             goto fail;