Likely incorrect patch: remove the faulty basename() prototype from libiberty

[DJ Delorie]

I think the right path to go down is to add libgen.h to the list in
AC_CHECK_HEADERS for every project that uses basename() (including
libiberty itself, and binutils, gdb, and gcc), and then everyone who
uses basename() from libiberty.h would need to include libgen.h *if*
it's found by configure.