Bug 11790: Remove dependency C4::Context from C4::Charset

Bug 11790: Remove dependency C4::Context from C4::Charset

C4::Context is only used to retrieve a syspref value.
This patch moves the use of C4::Context to a require.

Test plan:
Try to reach the SetMarcUnicodeFlag subroutine (batchmod, add/update a biblio, etc.)

Signed-off-by: Bernardo Gonzalez Kriegel <bgkriegel@gmail.com>
Tested on French UNIMARC install
No errors adding/editing biblios
No koha-qa errors

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>