test_intra_pred_speed: reuse test_libvpx's main()

this allows test_libvpx's simd caps check to be used; it also fixes a
link error on OS X with -fcommon.

Change-Id: I1a62a3e74ba06b8f3b37a22fcfdebf90c04ab289

test/test_intra_pred_speed.cc

@@ -332,4 +332,4 @@ INTRA_PRED_TEST(NEON, TestIntraPred32, NULL, NULL, NULL, NULL,
                 NULL, NULL, NULL, NULL, NULL, vp9_tm_predictor_32x32_neon)
 #endif  // HAVE_NEON
 
-#include "third_party/googletest/src/src/gtest_main.cc"
+#include "test/test_libvpx.cc"