Utils: Import std::experimental::in_place to Utils namespace

It is needed to construct an optional in place.

Change-Id: I92e6bec6a33b469d806d3144ea9eb8c64d25580f
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

src/libs/utils/optional.h

@@ -41,6 +41,8 @@ namespace Utils {
 using std::experimental::optional;
 // --> Utils::nullopt
 using std::experimental::nullopt;
+// --> Utils::inplace
+using std::experimental::in_place;
 
 // TODO: make_optional is a copy, since there is no sensible way to import functions in C++
 template <class T>