kbuild: setlocalversion: print error to STDERR

I tried to use 'make O=...' from an unclean source tree. This triggered
the error path of setlocalversion. But by printing to STDOUT, it created
a broken localversion which then caused another (unrelated) error:

"4.7.0-rc2Error: kernelrelease not valid - run make prepare to update it" exceeds 64 characters

After printing to STDERR, the true build error gets displayed later:

  /home/wsa/Kernel/linux is not clean, please run 'make mrproper'
  in the '/home/wsa/Kernel/linux' directory.

Signed-off-by: Wolfram Sang <wsa@the-dreams.de>
Signed-off-by: Michal Marek <mmarek@suse.com>