Fix a template that will break with GCC 11.x.

Change-Id: I7173f86affd7dc01f1b2d870c1305c5867f2a9ac
Reviewed-on: https://code-review.googlesource.com/c/re2/+/57810
Reviewed-by: Paul Wankadia <junyer@google.com>

re2/walker-inl.h

@@ -119,7 +119,7 @@ template<typename T> T Regexp::Walker<T>::Copy(T arg) {
 
 // State about a single level in the traversal.
 template<typename T> struct WalkState {
-  WalkState<T>(Regexp* re, T parent)
+  WalkState(Regexp* re, T parent)
     : re(re),
       n(-1),
       parent_arg(parent),