vp8_change_config: fix unsigned/signed comparison

Use ~15 instead of 0x..F0

Cleans warning in Android build:
comparison of integers of different signs: 'unsigned int' and 'int'
if (((cm->Width + 15) & 0xfffffff0) !=
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  ^
comparison of integers of different signs: 'unsigned int' and 'int'
((cm->Height + 15) & 0xfffffff0) !=
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  ^

Change-Id: Iac25839cde3425b7b9db7f33740dc46a551b7546