Fix C compatibility in header.

This struct is later used without the struct keyword, so it's not
valid C.

Test: treehugger
Bug: https://github.com/chadversary/android-ndk-rutabagas/blob/f3b3cd3de6b49f33ad82c1318c2ee280e5231c8d/src/main/ru_media.c#L41
Change-Id: I6ecd0614815749c0c17b8327bde6c8035fbd9932