cmake: Silence ranlib warnings when jnt_comp is disabled.

Build aom_dsp/x86/variance_ssse3.c and
av1/common/x86/convolve_2d_sse4.c only when jnt_comp is enabled.

When jnt_comp is disabled the sources define no symbols and
ranlib complains.

Change-Id: I3de42be6a88bd65459799d3a523e307c40a36a72
diff --git a/aom_dsp/aom_dsp.cmake b/aom_dsp/aom_dsp.cmake
index 4567f6e..8fcfbcb 100644
--- a/aom_dsp/aom_dsp.cmake
+++ b/aom_dsp/aom_dsp.cmake
@@ -59,8 +59,13 @@
 set(AOM_DSP_COMMON_INTRIN_SSSE3
     "${AOM_ROOT}/aom_dsp/x86/aom_subpixel_8t_intrin_ssse3.c"
     "${AOM_ROOT}/aom_dsp/x86/intrapred_ssse3.c"
-    "${AOM_ROOT}/aom_dsp/x86/inv_txfm_ssse3.c"
-    "${AOM_ROOT}/aom_dsp/x86/variance_ssse3.c")
+    "${AOM_ROOT}/aom_dsp/x86/inv_txfm_ssse3.c")
+
+if (CONFIG_JNT_COMP)
+  set(AOM_DSP_COMMON_INTRIN_SSSE3
+      ${AOM_DSP_COMMON_INTRIN_SSSE3}
+      "${AOM_ROOT}/aom_dsp/x86/variance_ssse3.c")
+endif ()
 
 set(AOM_DSP_COMMON_INTRIN_SSE4_1
     "${AOM_ROOT}/aom_dsp/x86/blend_a64_hmask_sse4.c"