fixes high-bitdepth build:
./libaom.a(aom_dsp_rtcd.c.o): In function `setup_rtcd_internal':
  ./aom_dsp_rtcd.h:2614: undefined reference to
  `aom_highbd_idct8x8_64_add_c'

missed in:
c756e4d0 Cleanup dead high-bitdepth inverse-tx functions

BUG=aomedia:442

Change-Id: I63ee6fc5dbf85fd48efd9ff721868df6fb05eb09