util: fix build on RHEL 6
util: fix build on RHEL 6

We keep forgetting that older setups don't like 'index':

  CC     util/libvirt_util_la-virsysinfo.lo
cc1: warnings being treated as errors
util/virstoragefile.c: In function 'virStorageSourceFindByNodeName':
util/virstoragefile.c:3804: error: declaration of 'index' shadows a global declaration [-Wshadow]
/usr/include/string.h:489: error: shadowed declaration is here [-Wshadow]

Signed-off-by: Eric Blake <a9da18479c871fe903d9590822cfe2629c6a4d11@redhat.com>

fabianfreyer/libvirt,zippy2/libvirt,nertpinx/libvirt,eskultety/libvirt,datto/libvirt,libvirt/libvirt,eskultety/libvirt,datto/libvirt,jardasgit/libvirt,eskultety/libvirt,zippy2/libvirt,crobinso/libvirt,olafhering/libvirt,jardasgit/libvirt,crobinso/libvirt,crobinso/libvirt,andreabolognani/libvirt,eskultety/libvirt,jardasgit/libvirt,nertpinx/libvirt,libvirt/libvirt,fabianfreyer/libvirt,libvirt/libvirt,olafhering/libvirt,andreabolognani/libvirt,VenkatDatta/libvirt,VenkatDatta/libvirt,fabianfreyer/libvirt,nertpinx/libvirt,fabianfreyer/libvirt,datto/libvirt,fabianfreyer/libvirt,jfehlig/libvirt,eskultety/libvirt,jfehlig/libvirt,jfehlig/libvirt,andreabolognani/libvirt,jardasgit/libvirt,nertpinx/libvirt,jardasgit/libvirt,VenkatDatta/libvirt,andreabolognani/libvirt,datto/libvirt,zippy2/libvirt,libvirt/libvirt,zippy2/libvirt,nertpinx/libvirt,VenkatDatta/libvirt,VenkatDatta/libvirt,crobinso/libvirt,jfehlig/libvirt,olafhering/libvirt,olafhering/libvirt,datto/libvirt,andreabolognani/libvirt