cosmetics: normalize include guards

use the recommended format [1] of:
<PROJECT>_<PATH>_<FILE>_H_

[1] https://google.github.io/styleguide/cppguide.html#The__define_Guard
"All header files should have #define guards to prevent multiple
inclusion. The format of the symbol name should be
<PROJECT>_<PATH>_<FILE>_H_."

Change-Id: I00a0595fc2a5fc736ab6ff3215727d6db05b10e1
diff --git a/av1/encoder/encoder.h b/av1/encoder/encoder.h
index 307e12d..54a26da 100644
--- a/av1/encoder/encoder.h
+++ b/av1/encoder/encoder.h
@@ -9,8 +9,8 @@
  * PATENTS file, you can obtain it at www.aomedia.org/license/patent.
  */
 
-#ifndef AV1_ENCODER_ENCODER_H_
-#define AV1_ENCODER_ENCODER_H_
+#ifndef AOM_AV1_ENCODER_ENCODER_H_
+#define AOM_AV1_ENCODER_ENCODER_H_
 
 #include <stdio.h>
 
@@ -980,4 +980,4 @@
 }  // extern "C"
 #endif
 
-#endif  // AV1_ENCODER_ENCODER_H_
+#endif  // AOM_AV1_ENCODER_ENCODER_H_