Compilation for mingw failed:

../../src/util/virutil.c: In function 'virGetWin32DirectoryRoot':
../../src/util/virutil.c:1094:9: error: unused variable 'ret' [-Werror=unused-variable]

* src/util/virutil.c (virGetWin32DirectoryRoot): Silence compiler
warning.
Signed-off-by: NEric Blake <eblake@redhat.com>