libxl: fix gentypes call in Makefile

libxl: fix gentypes call in Makefile

From the make documentation:

"$* [...] If the target is `dir/a.foo.b' and the target pattern is
`a.%.b' then the stem is `dir/foo'. In a static pattern rule, the
stem is part of the file name that matched the `%' in the target
pattern."

The rule generating the c types files from the idl ones is not
a static pattern rule, but rather an implicit rule. Thus the value
of $* is preceded by the file path, instead of only what matches %.

In order to get this fixed, drop the path using a $(notdir $*).

Signed-off-by: Cédric Bosdonnat <cbosdonnat@suse.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>