ld/plugins: avoid shadowing a C library symbol

ld/plugins: avoid shadowing a C library symbol

With my (oldish) gcc/glibc combination I'm seeing

.../ld/plugin.c: In function \91get_lto_kind\92:
.../ld/plugin.c:664: error: declaration of \91index\92 shadows a global declaration
/usr/include/string.h:487: error: shadowed declaration is here
.../ld/plugin.c: In function \91get_lto_resolution\92:
.../ld/plugin.c:685: error: declaration of \91index\92 shadows a global declaration
/usr/include/string.h:487: error: shadowed declaration is here
.../ld/plugin.c: In function \91get_lto_visibility\92:
.../ld/plugin.c:711: error: declaration of \91index\92 shadows a global declaration
/usr/include/string.h:487: error: shadowed declaration is here