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: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
diff --git a/.gitignore b/.gitignore
index 59c343c..6d2acab 100644
--- a/.gitignore
+++ b/.gitignore
@@ -15,6 +15,7 @@
 libhw32
 libhw64
 libuser
+linux-headers/asm
 qapi-generated
 qemu-doc.html
 qemu-tech.html