Fix linker warnings for bilinear filters

The declaration of the bilinear filters specified an alignment clause
in the implementation file but not in the header.  This turned out
to be harmless, but it did cause linker warnings to be emitted when
building on Windows.

The (extern) declaration in the header was changed, to match the
declaration in the implementation.

Change-Id: I44be89b1572fe9a50fa47a42e4db9128c4897b04