Mark potentially unused variable as maybe_unused

Fixes compile error with MinGW clang.

Amends de6f405313

Pick-to: 6.6 6.5
Change-Id: Idc1fac9d92369cfc79b5c770830bb43b3c5aae91
Reviewed-by: Marc Mutz <marc.mutz@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/global/qlibraryinfo.cpp

@@ -480,6 +480,7 @@ QLibraryInfoPrivate::LocationInfo QLibraryInfoPrivate::locationInfo(QLibraryInfo
         "Examples", "examples",
         "Tests", "tests"
     );
+    [[maybe_unused]]
     constexpr QByteArrayView dot{"."};
 
     LocationInfo result;