cifs-utils: fix probabilistic compiling error

cifs-utils: fix probabilistic compiling error

When we compile cifs-utils, we may probabilistic
encounter install error like:
cd ***/sbin && ln -sf mount.cifs mount.smb3
***/sbin: No such file or directory

The reason of this problem is that if we compile
cifs-utils using multithreading, target
'install-sbinPROGRAMS' may be built after
target 'install-exec-hook' of the main Makefile.
Target 'install-sbinPROGRAMS' will copy the
executable file 'mount.cifs' to the $(ROOTSBINDIR),
which target 'install-exec-hook' will do the
'ln' command on.

This patch add the dependency of target
'install-exec-hook' to ensure the correct order
of the compiling.

Signed-off-by: lizhe <lizhe67@huawei.com>