eval: Explicitly ignore return value of strtod() in parse_db()

The return value is not needed.  This fixes the warning:
libavutil/eval.c:353:15: warning: ignoring return value of ‘strtod’

libavutil/eval.c

@@ -26,6 +26,7 @@
  * see http://joe.hotchkiss.com/programming/eval/eval.html
  */
 
+#include "attributes.h"
 #include "avutil.h"
 #include "common.h"
 #include "eval.h"
@@ -350,7 +351,7 @@ static int parse_dB(AVExpr **e, Parser *p, int *sign)
        for example, -3dB is not the same as -(3dB) */
     if (*p->s == '-') {
         char *next;
-        strtod(p->s, &next);
+        double av_unused ignored = strtod(p->s, &next);
         if (next != p->s && next[0] == 'd' && next[1] == 'B') {
             *sign = 0;
             return parse_primary(e, p);