Add std namespace to string to fix build

These are currently unprefixed which fails with bazel build.

With this change
$ bazel build -- //...  -//integrations/tensorflow/e2e:simple_arithmetic_test
passes (exclude is for the failures in #127)

Closes #128

COPYBARA_INTEGRATE_REVIEW=https://github.com/google/iree/pull/128 from google:gcmn 5a6e6800ab5309e1f01342bb839f7a6165d2ce79
PiperOrigin-RevId: 279414305
diff --git a/build_tools/embed_data/testembed1_test.cc b/build_tools/embed_data/testembed1_test.cc
index a1c815f..43df0f1 100644
--- a/build_tools/embed_data/testembed1_test.cc
+++ b/build_tools/embed_data/testembed1_test.cc
@@ -20,18 +20,18 @@
 
 TEST(Generator, TestContents) {
   auto* toc = ::foobar::testembed1_create();
-  ASSERT_EQ("file1.txt", string(toc->name));
+  ASSERT_EQ("file1.txt", std::string(toc->name));
   ASSERT_EQ(R"(Are you '"Still"' here?)"
             "\n",
-            string(toc->data));
+            std::string(toc->data));
   ASSERT_EQ(24, toc->size);
   ASSERT_EQ(0, *(toc->data + toc->size));
 
   ++toc;
-  ASSERT_EQ("file2.txt", string(toc->name));
+  ASSERT_EQ("file2.txt", std::string(toc->name));
   ASSERT_EQ(R"(¯\_(ツ)_/¯)"
             "\n",
-            string(toc->data));
+            std::string(toc->data));
   ASSERT_EQ(14, toc->size);
   ASSERT_EQ(0, *(toc->data + toc->size));