libv4l2.h uses #ifdef, not #if

diff --git a/libavdevice/v4l2.c b/libavdevice/v4l2.c
index 42d4b97c8f701..0ae68723382f4 100644
--- a/libavdevice/v4l2.c
+++ b/libavdevice/v4l2.c
@@ -47,6 +47,10 @@
 #include "v4l2-common.h"
 #include <dirent.h>
 
+#if ! HAVE_POSIX_IOCTL
+#undef HAVE_POSIX_IOCTL
+#endif
+
 #if CONFIG_LIBV4L2
 #include <libv4l2.h>
 #endif