Mark callbackParallelThreadsDefault properly

The definition gets epicsShareDef and may provide a value.
Additional declarations get epicsShareExtern.

src/ioc/db/callback.c

@@ -63,7 +63,7 @@ static cbQueueSet callbackQueue[NUM_CALLBACK_PRIORITIES];
 int callbackThreadsDefault = 1;
 /* Don't know what a reasonable default is (yet).
  * For the time being: parallel means 2 if not explicitly specified */
-int callbackParallelThreadsDefault = 2;
+epicsShareDef int callbackParallelThreadsDefault = 2;
 epicsExportAddress(int,callbackParallelThreadsDefault);
 
 /* Timer for Delayed Requests */

src/ioc/db/dbIocRegister.c

@@ -23,7 +23,7 @@
 #include "dbIocRegister.h"
 #include "dbState.h"
 
-epicsShareDef int callbackParallelThreadsDefault;
+epicsShareExtern int callbackParallelThreadsDefault;
 
 /* dbLoadDatabase */
 static const iocshArg dbLoadDatabaseArg0 = { "file name",iocshArgString};