setup.py: add 6.4 to vmtest kernels

6.4 needed an unusual number of changes:

- 3f3a957 ("libdrgn: linux_kernel: Fix module detection on kernel
  v6.4")
- da7a11f ("drgn.helpers.linux.block: update
  for_each_{disk,partition} for Linux 6.4")
- 7e7c300a139b ("libdrgn: orc_info: handle ORC changes in Linux 6.3 and
  6.4")

Signed-off-by: Omar Sandoval <osandov@osandov.com>

setup.py

@@ -131,6 +131,7 @@ class test(Command):
     description = "run unit tests after in-place build"
 
     KERNELS = [
+        "6.4",
         "6.3",
         "6.2",
         "6.1",