readelf: Don't shadow index function from string.h

readelf: Don't shadow index function from string.h

On some ancient GCC versions (4.4.7 at least) -Wshadow warns about local
variables "shadowing" global function definitions.

  readelf.c: In function ‘print_debug_addr_section’:
  readelf.c:5265: error: declaration of ‘index’ shadows a global declaration
  /usr/include/string.h:489: error: shadowed declaration is here

This is silly of course, but easy to work around.

Signed-off-by: Mark Wielaard <mark@klomp.org>