Add auto-gen SM2 der files into .gitignore

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/12536)

.gitignore

@@ -57,12 +57,14 @@ providers/common/der/der_ec_gen.c
 providers/common/der/der_ecx_gen.c
 providers/common/der/der_rsa_gen.c
 providers/common/der/der_wrap_gen.c
+providers/common/der/der_sm2_gen.c
 providers/common/include/prov/der_dsa.h
 providers/common/include/prov/der_ec.h
 providers/common/include/prov/der_ecx.h
 providers/common/include/prov/der_rsa.h
 providers/common/include/prov/der_digests.h
 providers/common/include/prov/der_wrap.h
+providers/common/include/prov/der_sm2.h
 
 # error code files
 /crypto/err/openssl.txt.old