base: sync: Include seq_file.h in sync.h

sync.h uses struct seq_file in some function table prototypes. This
causes compile failures when including the header in files that do not
otherwise include seq_file.h

Change-Id: Icd294544c54bf11f44bbb90fa0900d1e6e22df9f
Signed-off-by: Jonathan Hamilton <jonathan.hamilton@imgtec.com>