Fix CMake build with CONFIG_AV1_ENCODER=0.

BUG=aomedia:708

Change-Id: I1ec236ae5010938f60d9f2b72e03f2db31eaf653

test/test.cmake

diff --git a/test/test.cmake b/test/test.cmake
index b035184..f939bc8 100644
--- a/test/test.cmake
+++ b/test/test.cmake
@@ -133,9 +133,12 @@
         "${AOM_ROOT}/test/av1_txfm_test.h"
         "${AOM_ROOT}/test/intrapred_test.cc"
         "${AOM_ROOT}/test/lpf_8_test.cc"
-        "${AOM_ROOT}/test/motion_vector_test.cc"
         "${AOM_ROOT}/test/simd_cmp_impl.h")
 
+    set(AOM_UNIT_TEST_ENCODER_SOURCES
+        ${AOM_UNIT_TEST_ENCODER_SOURCES}
+        "${AOM_ROOT}/test/motion_vector_test.cc")
+
     if (CONFIG_CDEF)
       if (CONFIG_CDEF_SINGLEPASS)
         set(AOM_UNIT_TEST_COMMON_SOURCES