Fix CONFIG_HIGHBITDEPTH in cmake.

Broken since 9d247355 when aom_dsp/x86/highbd_convolve_avx2.c was
added to aom_dsp.mk.

Change-Id: Ide6779209a546e1bf84a4997c0cdcf3b2bc2b92b
diff --git a/aom_dsp/aom_dsp.cmake b/aom_dsp/aom_dsp.cmake
index bed72d2..5137e06 100644
--- a/aom_dsp/aom_dsp.cmake
+++ b/aom_dsp/aom_dsp.cmake
@@ -180,6 +180,10 @@
   set(AOM_DSP_COMMON_INTRIN_SSE2
       ${AOM_DSP_COMMON_INTRIN_SSE2}
       "${AOM_ROOT}/aom_dsp/x86/highbd_loopfilter_sse2.c")
+
+  set(AOM_DSP_COMMON_INTRIN_AVX2
+      ${AOM_DSP_COMMON_INTRIN_AVX2}
+      "${AOM_ROOT}/aom_dsp/x86/highbd_convolve_avx2.c")
 else ()
   set(AOM_DSP_COMMON_INTRIN_DSPR2
       ${AOM_DSP_COMMON_INTRIN_DSPR2}