SELinux: build fix for 4.1

Commit "SELinux: Enable setting security contexts on rootfs inodes."
cherry-picked in experimental/android-4.1 used a now obsolete flag
SE_SBLABELSUPP. Rename it to SBLABEL_MNT as intended by upstream
commit 12f348b9 "SELinux: rename SE_SBLABELSUPP to SBLABEL_MNT",
otherwise we run into following build error:

  CC      security/selinux/hooks.o
security/selinux/hooks.c: In function ‘sb_finish_set_opts’:
security/selinux/hooks.c:459:19: error: ‘SE_SBLABELSUPP’ undeclared (first use in this function)
   sbsec->flags |= SE_SBLABELSUPP;
                   ^
security/selinux/hooks.c:459:19: note: each undeclared identifier is reported only once for each function it appears in
make[2]: *** [security/selinux/hooks.o] Error 1

Signed-off-by: Amit Pundir <amit.pundir@linaro.org>