mtd: remove put_partition_parser() from public header

There is no need to pollute public header with a definition private
to mtdpart.c. Move it from mtd/partitions.h to mtdpart.c

Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>