Make argument const char * as per Takahiro Itagaki's suggestion.

src/interfaces/ecpg/ecpglib/prepare.c

-/* $PostgreSQL: pgsql/src/interfaces/ecpg/ecpglib/prepare.c,v 1.35 2010/01/22 14:13:03 meskes Exp $ */
+/* $PostgreSQL: pgsql/src/interfaces/ecpg/ecpglib/prepare.c,v 1.36 2010/01/22 14:19:27 meskes Exp $ */
 
 #define POSTGRES_ECPG_INTERNAL
 #include "postgres_fe.h"
@@ -414,7 +414,7 @@ ecpg_freeStmtCacheEntry(int lineno, int compat, int entNo)		/* entry # to free *
  */
 static int
 AddStmtToCache(int lineno,		/* line # of statement		*/
-			   char *stmtID,	/* statement ID				*/
+			   const char *stmtID,	/* statement ID				*/
 			   const char *connection,	/* connection				*/
 			   int compat,		/* compatibility level */
 			   const char *ecpgQuery)	/* query					*/