Clang: Remove undefined behavior

Never forward declare STL container.

Change-Id: Ie2d222f858f555294581bbea90300755973cb54e
Reviewed-by: Christian Stenger <christian.stenger@qt.io>
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

src/libs/utils/smallstring.h

@@ -44,6 +44,7 @@
 #include <cstring>
 #include <iosfwd>
 #include <string>
+#include <unordered_map>
 #include <utility>
 
 #pragma push_macro("constexpr")
@@ -842,14 +843,6 @@ template<> struct hash<Utils::SmallString>
     }
 };
 
-template<
-    class Key,
-    class T,
-    class Hash,
-    class KeyEqual,
-    class Allocator>
-class unordered_map;
-
 template<typename Key,
          typename Value,
          typename Hash = hash<Key>,