avisynth: Cast to the right type when loading avisynth library functions

Fixes a number of related warnings.

libavformat/avisynth.c

@@ -127,7 +127,8 @@ static av_cold int avisynth_load_library(void)
         return AVERROR_UNKNOWN;
 
 #define LOAD_AVS_FUNC(name, continue_on_fail)                          \
-        avs_library.name = GetProcAddress(avs_library.library, #name); \
+        avs_library.name = (name ## _func)                             \
+                           GetProcAddress(avs_library.library, #name); \
         if (!continue_on_fail && !avs_library.name)                    \
             goto fail;