configure: fix valgrind-massif to actually work

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

configure

@@ -2517,7 +2517,7 @@ case "$toolchain" in
     ;;
     valgrind-massif)
         target_exec_default="valgrind"
-        target_exec_args="--alloc-fn=av_malloc --alloc-fn=av_mallocz"
+        target_exec_args="--tool=massif --alloc-fn=av_malloc --alloc-fn=av_mallocz"
     ;;
     valgrind-memcheck)
         target_exec_default=${valgrind:-"valgrind"}