pinky,who: remove redeclarations of ttyname

pinky,who: remove redeclarations of ttyname

Given that `ttyname` is already conditionally declared in src/system.h,
other declarations are redundant and problematic for example in ChromeOS
which has a new FORTIFY implementation that, as an artifact of how it's
implemented, causes the compiler to complain if certain standard library
functions are redeclared without special compiler-specific attributes.

* src/pinky.c: Remove declaration (which was unused anyway).
* src/who.c: Remove declaration.
* src/system.h (getlogin, getuid, geteuid, getgrgid, getpwuid, ttyname):
Add the parameter type to the declarations to avoid warnings when these
backup declarations are used.