Parenthesize the usage of the macro argument within vec_search() macro definition

Change-Id: I488d7c2b864c0e3661c8abf0363e4b97984d4974
Signed-off-by: Andrew Yourtchenko <ayourtch@gmail.com>
diff --git a/src/vppinfra/vec.h b/src/vppinfra/vec.h
index 1994d81..de90abf 100644
--- a/src/vppinfra/vec.h
+++ b/src/vppinfra/vec.h
@@ -943,7 +943,7 @@
   word _v(i) = 0;					\
   while (_v(i) < vec_len(v))				\
   {							\
-    if (v[_v(i)] == E)				        \
+    if ((v)[_v(i)] == E)				        \
       break;						\
     _v(i)++;						\
   }							\