Fix mingw32 build

Don't define qemu_chr_open_eventfd() on Windows.

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
diff --git a/qemu-char.c b/qemu-char.c
index 6a3952c..33f2237 100644
--- a/qemu-char.c
+++ b/qemu-char.c
@@ -2087,11 +2087,12 @@
     }
 }
 
-CharDriverState *qemu_chr_open_eventfd(int eventfd){
-
+#ifndef _WIN32
+CharDriverState *qemu_chr_open_eventfd(int eventfd)
+{
     return qemu_chr_open_fd(eventfd, eventfd);
-
 }
+#endif
 
 static void tcp_chr_connect(void *opaque)
 {