argp: better 'inline'

argp: better 'inline'

Use extern-inline module to declare extern inline functions.
This avoids some bogus warning diagnostics.  Problem discovered
when modifying GNU tar to use the manywarnings module.
* lib/argp.h, lib/argp-xinl.c (ARGP_EI) [!_LIBC]:
* lib/argp-fmtstream.h, lib/argp-fs-xinl.c (ARGP_FS_EI) [!_LIBC]:
Define based on extern-inline.
* modules/argp (Depends-on): Add extern-inline.