avfilter/f_sidedata: fix Wtautological-constant-out-of-range-compare

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavfilter/f_sidedata.c

@@ -39,7 +39,7 @@ typedef struct SideDataContext {
     const AVClass *class;
 
     int mode;
-    enum AVFrameSideDataType type;
+    int type;   // enum AVFrameSideDataType or -1 for delete side data mode
 } SideDataContext;
 
 #define OFFSET(x) offsetof(SideDataContext, x)