maint: correct quoting for gl_WARN_ADD
maint: correct quoting for gl_WARN_ADD

3 out of 4 uses of gl_WARN_ADD() were incorrectly adding "" around
the argument, which in turn resulted in the argument being used
unquoted (configure had gl_positive=""-fstack-protector-all"",
rather than the intended gl_positive="-fstack-protector-all").

Signed-off-by: Eric Blake <a9da18479c871fe903d9590822cfe2629c6a4d11@redhat.com>
Reviewed-by: Andrea Bolognani <3ada0bee826c753786fdbba72243dfba997094cf@redhat.com>

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