Mark loopfilter C functions as static

Clang defaults to C99 mode, and inline works differently in C99.
(gcc, on the other hand, defaults to a special gnu-style inlining,
which uses different syntax.)   Making the functions static makes sure
clang doesn't decide to discard a function because it's too large to
inline.

Thanks to eli.friedman for the patch.

Fixes http://code.google.com/p/webm/issues/detail?id=114

Change-Id: If3c1c3c176eb855a584a60007237283b0cc631a4