pidl: Fix a const build warning in the samba3 merged build

pidl: Fix a const build warning in the samba3 merged build

The warning:
../librpc/gen_ndr/ndr_spoolss.c: In function ‘ndr_pull_spoolss_EnumPrinterData’:
../librpc/gen_ndr/ndr_spoolss.c:14792: warning: passing argument 1 of ‘memset’ discards qualifiers from pointer target type

In some case the pidl generated code needs to memset a const struct
field.  This causes the above warning.  This patch discardeds the const
in this case.  I'm open to a more elegant fix, but this eliminates the
warning and should be relatively safe.