gitignore: Ignore generated files from libtasn

The commit 504058e8 (libtasn1: Compile into asn1 module) generates files
into the grub-core/lib/libtasn1-grub directory and commit 99cda678
(asn1_test: Test module for libtasn1) generates files into the
grub-core/tests/asn1/tests directory. Ignore these directories as they
are not under revision control.

Signed-off-by: Glenn Washburn <development@efficientek.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

.gitignore

@@ -141,10 +141,12 @@ widthspec.bin
 /grub-core/kernel.img.bin
 /grub-core/lib/gnulib
 /grub-core/lib/libgcrypt-grub
+/grub-core/lib/libtasn1-grub
 /grub-core/modinfo.sh
 /grub-core/rs_decoder.h
 /grub-core/symlist.c
 /grub-core/symlist.h
+/grub-core/tests/asn1/tests
 /grub-core/trigtables.c
 /grub-core/unidata.c
 /grub-editenv