doxygen: Add av_alloc_size to list of predefined macros

This avoids Doxygen believing the attribute is the function name.

doc/Doxyfile

@@ -1375,7 +1375,8 @@ INCLUDE_FILE_PATTERNS  =
 
 PREDEFINED             = "__attribute__(x)=" \
                          "DECLARE_ALIGNED(a,t,n)=t n" \
-                         "offsetof(x,y)=0x42"
+                         "offsetof(x,y)=0x42" \
+                         av_alloc_size \
 
 # If the MACRO_EXPANSION and EXPAND_ONLY_PREDEF tags are set to YES then
 # this tag can be used to specify a list of macro names that should be expanded.