build: fix build with optimization enabled

Build failure detected by Jenkins autobuilder:
http://honk.sigxcpu.org:8001/job/libvirt-build/472/changes

* .gnulib: Update to latest, for stdio.h rpl_fwrite fix.

.gnulib @ 61c7b1e3

-Subproject commit 964bbc2d419584e93fe629ddbc40595612f62083
+Subproject commit 61c7b1e32e11e9e40b4d59ab888a807620befcd3