Fix the build failure
Fix the build failure

Commit 16251193afdc forgot to include "virstring.h". Pushed under
build-breaker rule.

agx/libvirt,crobinso/libvirt,eskultety/libvirt,zhlcindy/libvirt-1.1.4-maintain,nertpinx/libvirt,jfehlig/libvirt,datto/libvirt,elmarco/libvirt,nertpinx/libvirt,cbosdo/libvirt,agx/libvirt,elmarco/libvirt,fabianfreyer/libvirt,zippy2/libvirt,taget/libvirt,rlaager/libvirt,fabianfreyer/libvirt,libvirt/libvirt,shugaoye/libvirt,shugaoye/libvirt,rlaager/libvirt,datto/libvirt,crobinso/libvirt,andreabolognani/libvirt,olafhering/libvirt,datto/libvirt,jardasgit/libvirt,zhlcindy/libvirt-1.1.4-maintain,fabianfreyer/libvirt,agx/libvirt,olafhering/libvirt,crobinso/libvirt,nertpinx/libvirt,rlaager/libvirt,taget/libvirt,andreabolognani/libvirt,andreabolognani/libvirt,eskultety/libvirt,libvirt/libvirt,VenkatDatta/libvirt,taget/libvirt,shugaoye/libvirt,taget/libvirt,jfehlig/libvirt,cbosdo/libvirt,olafhering/libvirt,crobinso/libvirt,elmarco/libvirt,eskultety/libvirt,eskultety/libvirt,zhlcindy/libvirt-1.1.4-maintain,fabianfreyer/libvirt,shugaoye/libvirt,elmarco/libvirt,VenkatDatta/libvirt,VenkatDatta/libvirt,fabianfreyer/libvirt,zippy2/libvirt,rlaager/libvirt,nertpinx/libvirt,jardasgit/libvirt,jardasgit/libvirt,nertpinx/libvirt,datto/libvirt,libvirt/libvirt,jardasgit/libvirt,zippy2/libvirt,taget/libvirt,jfehlig/libvirt,cbosdo/libvirt,datto/libvirt,andreabolognani/libvirt,rlaager/libvirt,zhlcindy/libvirt-1.1.4-maintain,eskultety/libvirt,cbosdo/libvirt,jfehlig/libvirt,VenkatDatta/libvirt,andreabolognani/libvirt,agx/libvirt,VenkatDatta/libvirt,shugaoye/libvirt,agx/libvirt,elmarco/libvirt,zippy2/libvirt,olafhering/libvirt,jardasgit/libvirt,libvirt/libvirt,zhlcindy/libvirt-1.1.4-maintain,cbosdo/libvirt