[Bug c++/87407] Enhance -Wunused-function to handle also inline functions

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87407

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
This is done on purpose as static inline functions are located in many headers
files including gcc's.