libfdisk: constify builtin fdisk_parttype

libfdisk: constify builtin fdisk_parttype

The builtin struct fdisk_parttypes are not supposed to be modified.
This is enforced in the mutation functions.

While the API could be cleaned up to distinguish between const and
non-const versions of fdisk_parttype this could break user code.

Instead preserve the API and add some internal casts.

Signed-off-by: Thomas Weißschuh <thomas@t-8ch.de>