Make version.texi depend on rust.texi.

This will at least update the version string on the title page
when the book source changes. It may not be entirely accurate
since rust.texi may have uncommitted changes. But previously,
it was basically only ever updated on the first build or after
'make clean'.

doc/Makefile

@@ -2,7 +2,7 @@ DOC_VER  := $(shell date +"%Y-%m-%d")-snap
 
 all: rust.pdf rust.html
 
-version.texi: Makefile
+version.texi: Makefile rust.texi
 	git log -1 \
       --format='@macro gitversion%n%h %ci%n@end macro%n' >$@