build: fix mingw build of virprocess.c

build: fix mingw build of virprocess.c

Commit 776d49f4 added a static function that is only called
conditionally; leading to this compile error on mingw:

  CC       libvirt_util_la-virprocess.lo
../../src/util/virprocess.c:624:26: error: 'struct rlimit' declared inside parameter list [-Werror]
../../src/util/virprocess.c:624:26: error: its scope is only this definition or declaration, which is probably not what you want [-Werror]
../../src/util/virprocess.c:622:1: error: 'virProcessPrLimit' defined but not used [-Werror=unused-function]

* src/util/virprocess.c (virProcessPrLimit): Only declare
virProcessPrLimit when used.

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