build: fix virutil build on mingw

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: NEric Blake <eblake@redhat.com>

src/util/virutil.c

@@ -1036,6 +1036,14 @@ virSetUIDGID(uid_t uid, gid_t gid, gid_t *groups ATTRIBUTE_UNUSED,
 
 #else /* ! HAVE_GETPWUID_R */
 
+int
+virGetGroupList(uid_t uid ATTRIBUTE_UNUSED, gid_t gid ATTRIBUTE_UNUSED,
+                gid_t **list)
+{
+    *list = NULL;
+    return 0;
+}
+
 # ifdef WIN32
 /* These methods are adapted from GLib2 under terms of LGPLv2+ */
 static int
@@ -1166,6 +1174,7 @@ virGetUserRuntimeDirectory(void)
 {
     return virGetUserCacheDirectory();
 }
+
 # else /* !HAVE_GETPWUID_R && !WIN32 */
 char *
 virGetUserDirectory(void)