decode: fix compile warning

decode.c:767:19: error: initializing 'char *' with an expression of type 'const char *' discards qualifiers
      [-Werror,-Wincompatible-pointer-types-discards-qualifiers]
  767 |             char *dot = strchr(DEvents[i].event_name, '.');
      |                   ^     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.

(cherry picked from commit a45a70babd)

src/decode.c

@@ -743,7 +743,7 @@ void DecodeRegisterPerfCounters(DecodeThreadVars *dtv, ThreadVars *tv)
             }
 
             char name[256];
-            char *dot = strchr(DEvents[i].event_name, '.');
+            const char *dot = strchr(DEvents[i].event_name, '.');
             BUG_ON(!dot);
             snprintf(name, sizeof(name), "%s.%s",
                     stats_decoder_events_prefix, dot+1);