target: target_core_ua_h: Add #define of include guard

Clearly the file was meant to contain an include guard, but it was
missing the #define part.
Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/target/target_core_ua.h

 #ifndef TARGET_CORE_UA_H
+#define TARGET_CORE_UA_H
 
 /*
  * From spc4r17, Table D.1: ASC and ASCQ Assignement