Add linux-headers/asm to .gitignore

linux-headers/asm is a symlink generated during configure.  It should not,
therefore be committed to git, nor show up in git diffs and the like.
Signed-off-by: NDavid Gibson <david@gibson.dropbear.id.au>
Reviewed-by: NJuan Quintela <quintela@redhat.com>
Signed-off-by: NBlue Swirl <blauwirbel@gmail.com>

.gitignore

@@ -15,6 +15,7 @@ libdis*
 libhw32
 libhw64
 libuser
+linux-headers/asm
 qapi-generated
 qemu-doc.html
 qemu-tech.html