tst_compiler: Silence MSVC 2015 warning about unused variable

tst_compiler.cpp(639): warning C4101: 'c': unreferenced local variable

Change-Id: If0101953ceb66bc8e8f5ab969d54f4e1c25dd414
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>

tests/auto/other/compiler/tst_compiler.cpp

@@ -637,6 +637,7 @@ void tst_Compiler::cxx11_alignas()
     QSKIP("Compiler does not support C++11 feature");
 #else
     alignas(double) char c;
+    Q_UNUSED(c);
     QCOMPARE(Q_ALIGNOF(c), Q_ALIGNOF(double));
 #endif
 }