lsm: constify 'bprm' parameter in security_bprm_committing_creds()

The 'bprm_committing_creds' hook has implementations registered in
SELinux and Apparmor. Looking at the function implementations we observe
that the 'bprm' parameter is not changing.

Mark the 'bprm' parameter of LSM hook security_bprm_committing_creds()
as 'const' since it will not be changing in the LSM hook.

Signed-off-by: Khadija Kamran <kamrankhadijadj@gmail.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>