Utils: Fix forgotten header file in qbs

Change-Id: I9dd079e3757f7026e188fab7798f3f4bb7ce30ec
Reviewed-by: Tim Jenssen <tim.jenssen@theqtcompany.com>

src/libs/utils/utils.qbs

@@ -187,6 +187,7 @@ QtcLibrary {
         "smallstring.h",
         "smallstringiterator.h",
         "smallstringliteral.h",
+        "smallstringlayout.h",
         "smallstringmemory.h",
         "smallstringvector.h",
         "statuslabel.cpp",