x86/hv: Add gitignore entry for generated header file

Commit 7bfe3b8e ("Drivers: hv: Introduce mshv_vtl driver") added a
new generated header file for the offsets into the mshv_vtl_cpu_context
structure to be used by the low-level assembly code.  But it didn't add
the .gitignore file to go with it, so 'git status' and friends will
mention it.

Let's add the gitignore file before somebody thinks that generated
header should be committed.

Fixes: 7bfe3b8e ("Drivers: hv: Introduce mshv_vtl driver")
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>