lsm: constify the 'file' parameter in security_binder_transfer_file()

SELinux registers the implementation for the "binder_transfer_file"
hook. Looking at the function implementation we observe that the
parameter "file" is not changing.

Mark the "file" parameter of LSM hook security_binder_transfer_file() as
"const" since it will not be changing in the LSM hook.

Signed-off-by: Khadija Kamran <kamrankhadijadj@gmail.com>
[PM: subject line whitespace fix]
Signed-off-by: Paul Moore <paul@paul-moore.com>