Hide the declaration of ffs() if an inline ffs() is implemented.

Hide the declaration of ffs() if an inline ffs() is implemented.

I couldn't find a better way to avoid compiler warnings about
redundant and/or inconsistent declaration of ffs().  I'd like to
be able to declare prototypes in general headers without committing
to implementing them as `static inline' or `extern', but there
seems to be no way to do this with gcc-2.6.1.  E.g.,

	int foo(void);
	static __inline int foo(void) { return 1; }

causes a warning about the linkage mismatch, while the opposite
order causes a warning about the redundant declaration.