Add missing filter.h to build system

Missing file causes 'make dist' to not include a complete copy of the
source.

Change-Id: I3f55aeb5a86d0e81234e4e4588cb8086ba4cfc4a

vp8/vp8_common.mk

@@ -24,6 +24,7 @@ VP8_COMMON_SRCS-yes += common/entropymode.c
 VP8_COMMON_SRCS-yes += common/entropymv.c
 VP8_COMMON_SRCS-yes += common/extend.c
 VP8_COMMON_SRCS-yes += common/filter.c
+VP8_COMMON_SRCS-yes += common/filter.h
 VP8_COMMON_SRCS-yes += common/findnearmv.c
 VP8_COMMON_SRCS-yes += common/generic/systemdependent.c
 VP8_COMMON_SRCS-yes += common/idctllm.c