build: fix virthread build on mingw

build: fix virthread build on mingw

Our recent conversion to make VIR_ALLOC report oom wasn't
tested on mingw:

In file included from ../../src/util/virthread.c:29:0:
../../src/util/virthreadwin32.c: In function 'virCondWait':
../../src/util/virthreadwin32.c:166:81: error: 'VIR_FROM_THIS' undeclared (first use in this function)
     if (VIR_REALLOC_N(c->waiters, c->nwaiters + 1) < 0) {
                                                                                 ^

* src/util/virthreadwin32.c (VIR_FROM_THIS): Define.

Signed-off-by: Eric Blake <eblake@redhat.com>