Guard header files in utils with #ifdef and #endif

Some header files in utils missing #ifdef and #endif and this caused including
header files multiple times.

This patch fix it.

BUG=dawn:72

Change-Id: Ib28b1a1cef91e49a5754ec6b3d66327076070d99
Reviewed-on: https://dawn-review.googlesource.com/c/3461
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>

src/utils/DawnHelpers.h

@@ -12,6 +12,9 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+#ifndef UTILS_DAWNHELPERS_H_
+#define UTILS_DAWNHELPERS_H_
+
 #include <dawn/dawncpp.h>
 
 #include <initializer_list>
@@ -98,3 +101,5 @@ namespace utils {
         std::initializer_list<BindingInitializationHelper> bindingsInitializer);
 
 }  // namespace utils
+
+#endif  // UTILS_DAWNHELPERS_H_

src/utils/SystemUtils.h

@@ -12,7 +12,12 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+#ifndef UTILS_SYSTEMUTILS_H_
+#define UTILS_SYSTEMUTILS_H_
+
 namespace utils {
 
     void USleep(unsigned int usecs);
 }
+
+#endif  // UTILS_SYSTEMUTILS_H_