missing.h: define IFLA_EXT_MASK

We already define IFLA_PROMISCUITY and some other of these masks in
order to allow building with older headers.  Define IFLA_EXT_MASK too,
which was added in the same kernel version as IFLA_PROMISCUITY (v3.10).

src/basic/missing.h

@@ -714,6 +714,7 @@ static inline int setns(int fd, int nstype) {
 #endif
 
 #if !HAVE_DECL_IFLA_PHYS_PORT_ID
+#define IFLA_EXT_MASK 29
 #undef IFLA_PROMISCUITY
 #define IFLA_PROMISCUITY 30
 #define IFLA_NUM_TX_QUEUES 31