Utils: Fix compile with gcc 4.9

Otherwise compile fails with
 constexpr constructor does not have empty body.
Broke with 2f8c075a01.

Change-Id: I04ce03eaf51ddb3b4233abc4b51a553355e28e04
Reviewed-by: Ivan Donchevskii <ivan.donchevskii@qt.io>

src/libs/utils/smallstringlayout.h

@@ -155,7 +155,6 @@ struct StringDataLayout {
                 : ((MaximumShortStringDataAreaSize + 2) % 16) == 0,
                   "Size + 1 must be dividable by 16 if under 64 and Size + 2 must be dividable by 16 if over 64!");
 
-    constexpr
     StringDataLayout() noexcept
     {
         reset();