lavu/avstring: deprecate av_d2str().

It is no longer used in our code base and does not seem
to be used much in other projects.

libavutil/avstring.c

@@ -136,6 +136,7 @@ end:
     return p;
 }
 
+#if FF_API_D2STR
 char *av_d2str(double d)
 {
     char *str = av_malloc(16);
@@ -143,6 +144,7 @@ char *av_d2str(double d)
         snprintf(str, 16, "%f", d);
     return str;
 }
+#endif
 
 #define WHITESPACES " \n\t\r"