Mention jit-dump.h in the documentation

I noticed that the documentation for the dumping functions did not
mention jit-dump.h.  This patch updates the documentation.

ChangeLog

@@ -1,3 +1,7 @@
+2018-01-15  Tom Tromey  <tom@tromey.com>
+
+	* jit/jit-dump.c: Document the need for jit-dump.h.
+
 2018-01-10  David Meyer  <pdox@alum.mit.edu>
 
 	* jit/jit-apply.c (jit_apply_builder_add_arguments): fix a bug with

jit/jit-dump.c

@@ -38,6 +38,10 @@
 
 @cindex jit-dump.h
 
+The library provides some functions for dumping various objects to a
+stream.  These functions are declared in the header
+@code{<jit/jit-dump.h>}.
+
 @*/
 
 /*@