samba4: add dependency on BR2_TOOLCHAIN_HAS_SYNC_4

samba4 uses the __sync_fetch_and_add_4() atomic built-in, so it should
depend on BR2_TOOLCHAIN_HAS_SYNC_4 in order to avoid build failures on
architectures not providing this atomic built-in.

Fixes:

  http://autobuild.buildroot.org/results/0d0fd9d2a132a40a840bea5df59c35d8573ebf45/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>