Introduce & use FASTCONVERTFUNC with full prototype

modules/database/src/std/link/lnkConst.c

@@ -23,8 +23,6 @@
 #include "epicsExport.h"
 
 
-typedef long (*FASTCONVERT)();
-
 typedef struct const_link {
     jlink jlink;        /* embedded object */
     int nElems;
@@ -458,7 +456,7 @@ static long lnkConst_loadArray(struct link *plink, short dbrType, void *pbuffer,
     short dbrSize;
     char *pdest = pbuffer;
     int nElems = clink->nElems;
-    FASTCONVERT conv;
+    FASTCONVERTFUNC conv;
     long status;
 
     if(INVALID_DB_REQ(dbrType))