pgsql-server: Make validate_exec take a const char *, for gcc 3.4, which

Log Message:
-----------
Make validate_exec take a const char *, for gcc 3.4, which is more
strict.

Modified Files:
--------------
pgsql-server/src/port:
exec.c (r1.13 -> r1.14)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/port/exec.c.diff?r1=1.13&r2=1.14)