Make omxtypes.h utf-8

Otherwise, Visual Studio warns:

c:\src\cr\src\third_party\openmax_dl\dl\api\omxtypes.h: warning C4828: The file contains a character starting at offset 0x70 that is illegal in the current source character set (codepage 65001).

when building with the /utf8 flag.

R=rtoy@google.com, rtoy@chromium.org
BUG=454858, 637203

Review URL: https://codereview.webrtc.org/2485303002 .