vp8dx.h: Add macro for skipping loop filter

Without this applications cannot use the vpx_codec_control macro
for VP9_SET_SKIP_LOOP_FILTER. The tests only cover the underscored
version vpx_codec_control_().

Change-Id: I3e6c1888307b76636fdc1a8deae70b5c14238163
(cherry picked from commit 373e08f921e5bfd5a96963fabbbbe16ec793d44e)
1 file changed