glsl/tests: Fix a compiler warning about signed/unsigned loop comparison.

Fixes: d32956935e ("glsl: Walk a list of ir_dereference_array to mark array elements as accessed")
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/compiler/glsl/tests/array_refcount_test.cpp

@@ -628,7 +628,7 @@ TEST_F(array_refcount_test, visit_array_indexing_an_array)
 
    ir_array_refcount_entry *const entry_c = v.get_variable_entry(var_c);
 
-   for (unsigned i = 0; i < var_c->type->array_size(); i++) {
+   for (int i = 0; i < var_c->type->array_size(); i++) {
       EXPECT_EQ(true, entry_c->is_linearized_index_referenced(i)) <<
          "array c, i = " << i;
    }