sbcl doesn implement clhs' recomendation for inlinable

[cbaggers]

The spec says:

To define a function f that is not inline by default but for which (declare (inline f)) will make f be locally inlined, the proper definition sequence is:

 (declaim (inline f))
 (defun f ...)
 (declaim (notinline f))

However in sbcl this gives

; caught STYLE-WARNING:
;   Call to CEPL.VIEWPORTS::%CURRENT-VIEWPORT could not be inlined because its
;   source code was not saved. A global INLINE or SB-EXT:MAYBE-INLINE proclamation
;   must be in effect to save function definitions for inlining.

change defn-inlinable to take this into account