Move build_module into an anonymous namespace

This CL moves build_module into an anonymous namespace which fixes up a
build warning about missing prototypes.

Change-Id: I8b2f427e32e7851c24492099103565ab0119f4fa
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/16503
Reviewed-by: Sarah Mashayekhi <sarahmashay@google.com>

src/validator_impl_import_test.cc

@@ -12,14 +12,16 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+#include <assert.h>
+
 #include <iostream>
+
 #include "gtest/gtest.h"
 #include "src/reader/wgsl/parser.h"
 #include "src/validator_impl.h"
 
 namespace tint {
-
-using ValidatorImplTest = testing::Test;
+namespace {
 
 ast::Module build_module(std::string data) {
   auto reader = std::make_unique<tint::reader::wgsl::Parser>(
@@ -28,6 +30,10 @@ ast::Module build_module(std::string data) {
   return reader->module();
 }
 
+}  // namespace
+
+using ValidatorImplTest = testing::Test;
+
 TEST_F(ValidatorImplTest, Import) {
   std::string input = "import \"GLSL.std.450\" as glsl;";
   auto module = build_module(input);