Alpha: Fix a missing comma in sys_osf_statfs()

Fix a comma that got accidentally deleted from sys_osf_statfs() leading to the
following warning:

  arch/alpha/kernel/osf_sys.c: In function 'SYSC_osf_statfs':
  arch/alpha/kernel/osf_sys.c:255: error: syntax error before 'buffer'

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>