iflib: Make iflib_stop() static
    
    It is declared as static. Make the definition consistent with the
    declaration.
    
    This follows 7ff9ae90f0bd and partially reverts 09f6ff4f1a47.
    
    Reviewed by:	erj
    MFC after:	1 week
    Differential Revision:	https://reviews.freebsd.org/D46185
    
    (cherry picked from commit 8161000892830ee52bc8048be91b40cdad25fea8)