build: fix virutil build on mingw

build: fix virutil build on mingw

https://bugzilla.redhat.com/show_bug.cgi?id=964358

On platforms without decent group support, the build failed:

Cannot export virGetGroupList: symbol not defined
./.libs/libvirt_security_manager.a(libvirt_security_manager_la-security_dac.o): In function `virSecurityDACPreFork':
/home/eblake/libvirt-tmp/build/src/../../src/security/security_dac.c:248: undefined reference to `virGetGroupList'
collect2: error: ld returned 1 exit status

* src/util/virutil.c (virGetGroupList): Provide dummy implementation.

Signed-off-by: Eric Blake <eblake@redhat.com>
(cherry picked from commit cd725c7a9350c8331a6f915127cf7007706f9926)