Utils: Fix unittest

Prevent operator selection to be ambiguous.

Change-Id: I1ff0d6aad8fe5ce24b0d1038c1b6121a595800fe
Reviewed-by: Christian Stenger <christian.stenger@qt.io>
Reviewed-by: David Schulz <david.schulz@qt.io>

tests/unit/unittest/smallstringtest.cpp

@@ -291,7 +291,7 @@ TEST(SmallString, BeginPlusOneIsEqualEndForSmallStringWidthSizeOne)
 {
     SmallString text("x");
 
-    auto beginPlusOne = text.begin() + 1l;
+    auto beginPlusOne = text.begin() + std::size_t(1);
 
     ASSERT_THAT(beginPlusOne, Eq(text.end()));
 }