selinux: removed redundant header from objsec.h and avc.h

flask.h was included in the above files. This is autogenerated header
which gets produced during compilation of selinux. This caused issues
for other module named PFK which includes objsec.h and compiles after
selinux. Since removal of the above headers does not break compilation,
they seem to be redundant and can be removed

Change-Id: Iaee034e09d58fc0c4c1670700f4a7751c9db708f
Signed-off-by: Andrey Markovytch <andreym@codeaurora.org>