sbc: Add a private boolean flag to enable 15 block encoding
sbc: Add a private boolean flag to enable 15 block encoding

This patch introduce a new private member which purpose is to encode 15
blocks. It is private to the library and can't be set from standard API.
sbc_init_msbc() function will be defined to set this flag.

pstglia/external-bluetooth-sbc,pstglia/external-bluetooth-sbc