Fix powerpc ifunc-sel.h build for -Os

[Joseph Myers]

On Tue, 27 Feb 2018, Sam Ravnborg wrote:

> It would be a little less verbose to use __always_inline from cdefs.h
> Note: I did not check if this is not feasible for some reasons

Some of the sources including ifunc-sel.h don't seem to include any system 
headers first (but I don't know if there's any particular reason they 
avoid system headers).

-- 
Joseph S. Myers
joseph@codesourcery.com