Change aom_codec_av1_cx_algo to aom_codec_av1_cx()

Replace &aom_codec_av1_cx_algo with aom_codec_av1_cx(). This fixes an
unresolved symbol error when doing a shared library build of libaom on
Windows. This is also recommended by the following comment in
aom/aomcx.h:

/*!\brief A single instance of the AV1 encoder.
 *\deprecated This access mechanism is provided for backwards compatibility;
 * prefer aom_codec_av1_cx().
 */
extern aom_codec_iface_t aom_codec_av1_cx_algo;

BUG=aomedia:3161

Change-Id: I0ab51b42a1cb68df253dd93fab5e83ad1aa0ecde
(cherry picked from commit 0fec265f34e91ee21c8f0efcd5f16113768980ec)

test/level_test.cc

diff --git a/test/level_test.cc b/test/level_test.cc
index f512c5a..972ade0 100644
--- a/test/level_test.cc
+++ b/test/level_test.cc
@@ -76,7 +76,7 @@
 };
 
 TEST_P(LevelTest, TestTargetLevelApi) {
-  static aom_codec_iface_t *codec = &aom_codec_av1_cx_algo;
+  static aom_codec_iface_t *codec = aom_codec_av1_cx();
   aom_codec_ctx_t enc;
   aom_codec_enc_cfg_t cfg;
   EXPECT_EQ(AOM_CODEC_OK, aom_codec_enc_config_default(codec, &cfg, 0));