fix building with --disable-spatial-resampling

vpx_scale.c is only used by the vp8 encoder when spatial resampling is
enabled.

Change-Id: If3d3ad81e9ee6e0b59f8c040b9624ef52598fe03

vpx_scale/vpx_scale.mk

 SCALE_SRCS-yes += vpx_scale.mk
 SCALE_SRCS-yes += yv12config.h
-SCALE_SRCS-yes += vpx_scale.h
-SCALE_SRCS-yes += generic/vpx_scale.c
+SCALE_SRCS-$(CONFIG_SPATIAL_RESAMPLING) += vpx_scale.h
+SCALE_SRCS-$(CONFIG_SPATIAL_RESAMPLING) += generic/vpx_scale.c
 SCALE_SRCS-yes += generic/yv12config.c
 SCALE_SRCS-yes += generic/yv12extend.c
 SCALE_SRCS-$(CONFIG_SPATIAL_RESAMPLING) += generic/gen_scalers.c