Make SPI_prepare argtypes argument const

This changes the argtypes argument of SPI_prepare(),
SPI_prepare_cursor(), SPI_cursor_open_with_args(), and
SPI_execute_with_args() from Oid *argtypes to const Oid *argtypes.
The underlying functions were already receptive to that, so this
doesn't require any significant changes beyond the function signatures
and some internal variables.

Commit 28972b6fc3dc recently introduced a case where a const had to be
cast away before calling these functions. This is fixed here.