tracing: Add missing syscall_metadata comment

Add the missing syscall_metadata description for the enter_fields
struct member.

Link: http://lkml.kernel.org/r/74c3407cd1e5d37f2c5aaca637aa4d35f66f1aa2.1372479499.git.tom.zanussi@linux.intel.com

Signed-off-by: Tom Zanussi <tom.zanussi@intel.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

include/trace/syscall.h

@@ -16,6 +16,7 @@
  * @nb_args: number of parameters it takes
  * @types: list of types as strings
  * @args: list of args as strings (args[i] matches types[i])
+ * @enter_fields: list of fields for syscall_enter trace event
  * @enter_event: associated syscall_enter trace event
  * @exit_event: associated syscall_exit trace event
  */