add .gitignore entries for *.[is] below xen

add .gitignore entries for *.[is] below xen

Instead of listing each single file with .s or .i suffixes in
.gitignore use pattern based entries. Restrict those to the xen
directory as we have e.g. tools/libs/stat/bindings/swig/xenstat.i in
our tree.

Below xen the pattern based entries are fine, as we have pattern rules
for creating *.s and *.i files in xen/Rules.mk.

Suggested-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Jan Beulich <jbeulich@suse.com>