vp9/neon: skip some files in high-bitdepth build

exclude files that only contain functions for non-high-bitdepth builds.
this removes some warnings related to missing prototypes

Change-Id: Ic6642998c46a7b808c6c53b2f9c34bcd4d037abe
diff --git a/vp9/vp9_common.mk b/vp9/vp9_common.mk
index c854983..4ed34d0 100644
--- a/vp9/vp9_common.mk
+++ b/vp9/vp9_common.mk
@@ -141,8 +141,10 @@
 VP9_COMMON_SRCS-$(HAVE_NEON_ASM) += common/arm/neon/vp9_mb_lpf_neon$(ASM)
 VP9_COMMON_SRCS-$(HAVE_NEON_ASM) += common/arm/neon/vp9_save_reg_neon$(ASM)
 
+ifneq ($(CONFIG_VP9_HIGHBITDEPTH),yes)
 VP9_COMMON_SRCS-$(HAVE_NEON) += common/arm/neon/vp9_iht4x4_add_neon.c
 VP9_COMMON_SRCS-$(HAVE_NEON) += common/arm/neon/vp9_iht8x8_add_neon.c
+endif
 VP9_COMMON_SRCS-$(HAVE_NEON) += common/arm/neon/vp9_loopfilter_neon.c
 
 # neon with assembly and intrinsics implementations. If both are available