missing: add SIOCGHWTSTAMP to missing.h

In order to allow hwtstamp_ctl to use the non-destructive SIOCGHWTSTAMP
ioctl, we need to add it to missing.h, in order to prevent build failure
on older kernels which don't have this support.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>

missing.h

@@ -50,6 +50,10 @@ enum _missing_hwtstamp_tx_types {
 };
 #endif
 
+#ifndef SIOCGHWTSTAMP
+#define SIOCGHWTSTAMP 0x89b1
+#endif
+
 #ifndef HAVE_CLOCK_ADJTIME
 static inline int clock_adjtime(clockid_t id, struct timex *tx)
 {