Fix brokenness on FreeBSD

[wahjava]

With sbcl on FreeBSD, (ql:quickload :woo) fails with:

; file: quicklisp/dists/quicklisp/software/woo-20151031-git/src/syscall/main.lisp
; in: DEFUN SENDFILE
;     (CFFI:WITH-FOREIGN-SLOTS (WOO.SYSCALL::SBYTES 'WOO.SYSCALL::OFF-T)
;       (LET ((WOO.SYSCALL::RETVAL
;              (WOO.SYSCALL::%SENDFILE WOO.SYSCALL::INFD WOO.SYSCALL::OUTFD
;               WOO.SYSCALL::OFFSET WOO.SYSCALL::NBYTES # WOO.SYSCALL::SBYTES
;               WOO.SYSCALL::+SF-MNOWAIT+)))
;         (DECLARE (TYPE FIXNUM WOO.SYSCALL::RETVAL))
;         (IF (= WOO.SYSCALL::RETVAL -1)
;             -1
;             (CFFI:MEM-AREF WOO.SYSCALL::SBYTES 'WOO.SYSCALL::OFF-T))))
; 
; caught ERROR:
;   during macroexpansion of
;   (CFFI:WITH-FOREIGN-SLOTS (SBYTES 'OFF-T)
;     (LET #
;       #
;       ...)).
;   Use *BREAK-ON-SIGNALS* to intercept.
;   
;    error while parsing arguments to DEFMACRO CFFI:WITH-FOREIGN-SLOTS:
;      too few elements in
;        (SBYTES 'OFF-T)
;      to satisfy lambda list
;        (CFFI::VARS CFFI::PTR TYPE):
;      exactly 3 expected, but got 2

debugger invoked on a UIOP/LISP-BUILD:COMPILE-FILE-ERROR in thread
#<THREAD "main thread" RUNNING {1002AE4A03}>:
  COMPILE-FILE-ERROR while
  compiling #<CL-SOURCE-FILE "woo" "src" "syscall" "main">

The attached commit diff fixes this.

Thanks!