mpi/ec: fix when 'unsigned long' is 32-bit but limb size is 64-bit

* mpi/ec.c (ec_addm_25519, ec_subm_25519, ec_mulm_25519): Cast '1' to
mpi_limb_t before left shift.

Patch fixes mpi/ec.c compiler warnings and failing tests cases on
Win64.

• Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>