avfilter/vf_histogram: Change enum to int, which is accessed via AVOption as int

This fixes depending on implementation defined behavior
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavfilter/vf_histogram.c

@@ -37,7 +37,7 @@ enum HistogramMode {
 
 typedef struct HistogramContext {
     const AVClass *class;               ///< AVClass context for log and options purpose
-    enum HistogramMode mode;
+    int mode;                           ///< HistogramMode
     unsigned       histogram[256];
     int            ncomp;
     const uint8_t  *bg_color;