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: NEric Blake <eblake@redhat.com>

src/util/virprocess.c

@@ -617,7 +617,7 @@ virProcessPrLimit(pid_t pid, int resource, struct rlimit *rlim)
 {
     return prlimit(pid, resource, rlim, NULL);
 }
-#else /* ! HAVE_PRLIMIT */
+#elif HAVE_SETRLIMIT
 static int
 virProcessPrLimit(pid_t pid ATTRIBUTE_UNUSED,
                   int resource ATTRIBUTE_UNUSED,
@@ -626,7 +626,7 @@ virProcessPrLimit(pid_t pid ATTRIBUTE_UNUSED,
     errno = ENOSYS;
     return -1;
 }
-#endif /* ! HAVE_PRLIMIT */
+#endif
 
 #if HAVE_SETRLIMIT && defined(RLIMIT_MEMLOCK)
 int