Centralize EC_MULTISYMBOL error checking.

The EC_ADAPT experiment cannot work unless EC_MULTISYMBOL is also
 enabled.
This patch replaces all individual checks with a centralized check in
 both the bitreader.h and bitwriter.h.

Change-Id: I418852d95c5012cc074ed65cd24997e08bc2aadd

aom_dsp/bitreader.h

diff --git a/aom_dsp/bitreader.h b/aom_dsp/bitreader.h
index 00ea158..478945b 100644
--- a/aom_dsp/bitreader.h
+++ b/aom_dsp/bitreader.h
@@ -16,6 +16,10 @@
 #include <limits.h>
 
 #include "./aom_config.h"
+#if CONFIG_EC_ADAPT && !CONFIG_EC_MULTISYMBOL
+#error "CONFIG_EC_ADAPT is enabled without enabling CONFIG_EC_MULTISYMBOL."
+#endif
+
 #include "aom/aomdx.h"
 #include "aom/aom_integer.h"
 #if CONFIG_ANS

aom_dsp/bitwriter.h

diff --git a/aom_dsp/bitwriter.h b/aom_dsp/bitwriter.h
index 7fcd9e9..ef529fc 100644
--- a/aom_dsp/bitwriter.h
+++ b/aom_dsp/bitwriter.h
@@ -14,6 +14,10 @@
 
 #include <assert.h>
 #include "./aom_config.h"
+#if CONFIG_EC_ADAPT && !CONFIG_EC_MULTISYMBOL
+#error "CONFIG_EC_ADAPT is enabled without enabling CONFIG_EC_MULTISYMBOL"
+#endif
+
 #if CONFIG_ANS
 #include "aom_dsp/buf_ans.h"
 #elif CONFIG_DAALA_EC