Relocate symbol exports needed by lightfield_bitstream_parsing.c

Move from aom/exports_test to aom/exports_enc:
aom_wb_write_literal
aom_wb_write_unsigned_literal

BUG=aomedia:2072

Change-Id: I73862a9aea282427419dcc0624cee54f31e78973
diff --git a/aom/exports_enc b/aom/exports_enc
index 918d742..1fa872d 100644
--- a/aom/exports_enc
+++ b/aom/exports_enc
@@ -16,3 +16,5 @@
 text aom_noise_model_get_grain_parameters
 text aom_noise_model_save_latest
 text aom_noise_model_update
+text aom_wb_write_literal
+text aom_wb_write_unsigned_literal
diff --git a/aom/exports_test b/aom/exports_test
index 5f2a121..01b864b 100644
--- a/aom/exports_test
+++ b/aom/exports_test
@@ -1,4 +1,2 @@
 text aom_dsp_rtcd
 text aom_scale_rtcd
-text aom_wb_write_literal
-text aom_wb_write_unsigned_literal