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/aom_dsp/entcode.h b/aom_dsp/entcode.h
index 5c15526..7ba2b1c 100644
--- a/aom_dsp/entcode.h
+++ b/aom_dsp/entcode.h
@@ -9,8 +9,8 @@
  * PATENTS file, you can obtain it at www.aomedia.org/license/patent.
  */
 
-#ifndef AOM_DSP_ENTCODE_H_
-#define AOM_DSP_ENTCODE_H_
+#ifndef AOM_AOM_DSP_ENTCODE_H_
+#define AOM_AOM_DSP_ENTCODE_H_
 
 #include <limits.h>
 #include <stddef.h>
@@ -37,4 +37,4 @@
 OD_WARN_UNUSED_RESULT uint32_t od_ec_tell_frac(uint32_t nbits_total,
                                                uint32_t rng);
 
-#endif  // AOM_DSP_ENTCODE_H_
+#endif  // AOM_AOM_DSP_ENTCODE_H_