AP_Common: add missing O_CLOEXEC definition

O_CLOEXEC is not available on Nuttx, nor it makes sense there. Define it
as 0, so we can use it in code.

libraries/AP_Common/missing/fcntl.h

@@ -0,0 +1,9 @@
+#include_next <fcntl.h>
+
+/*
+ * we only want to define as 0 for those targets in which it doesn't make
+ * sense
+ */
+#if defined(HAVE_OCLOEXEC) && HAVE_OCLOEXEC == 0
+#define O_CLOEXEC 0
+#endif