Add DEBUG_DROP_MONITOR table to schema.h (#971)

Added the DEBUG_DROP_MONITOR table definition to sonic-swss-common.

This change is a prerequisite for another PR (sonic-net/sonic-swss#3509) that relies on using the DEBUG_DROP_MONITOR table in the orchagent for persistent drop counter monitoring.

Author: arista-hpandya

common/schema.h

@@ -432,6 +432,7 @@ namespace swss {
 
 #define CFG_DEBUG_COUNTER_TABLE_NAME                "DEBUG_COUNTER"
 #define CFG_DEBUG_COUNTER_DROP_REASON_TABLE_NAME    "DEBUG_COUNTER_DROP_REASON"
+#define CFG_DEBUG_DROP_MONITOR_TABLE_NAME           "DEBUG_DROP_MONITOR"
 
 #define CFG_STATIC_NAT_TABLE_NAME                   "STATIC_NAT"
 #define CFG_STATIC_NAPT_TABLE_NAME                  "STATIC_NAPT"