[Patch] Fix enum type mismatch

Attached patch fixed following warning:

"../../../src/include/nodes/parsenodes.h", line 487: warning: enumerator
value overflows INT_MAX (2147483647)

The reason is clear, enum is int not unsigned.

It is short fix, but I'm thinking about enum conversion to #define. We
use e.g. in the same file.

60  typedef uint32 AclMode;    /* a bitmask of privilege bits */
61
62  #define ACL_INSERT        (1<<0)    /* for relations */
63  #define ACL_SELECT        (1<<1)
64  #define ACL_UPDATE        (1<<2)
65  #define ACL_DELETE        (1<<3)
66  #define ACL_TRUNCATE    (1<<4)
67  #define ACL_REFERENCES    (1<<5)
68  #define ACL_TRIGGER        (1<<6)

    Zdenek