Const-correct a few X509_PURPOSE and X509_TRUST functions

These bits need more work (and possibly some removal) as they're very,
very far from thread-safe, but rust-openssl relies on them being
const-correct when targetting OpenSSL 1.1.x.

Change-Id: I60531c7e90dbdbcb79c09fc440bd7c6b474172df
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/60607
Auto-Submit: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: Bob Beck <bbe@google.com>
Commit-Queue: Bob Beck <bbe@google.com>
diff --git a/include/openssl/x509.h b/include/openssl/x509.h
index b2ac3db..c41b3a5 100644
--- a/include/openssl/x509.h
+++ b/include/openssl/x509.h
@@ -2529,7 +2529,7 @@
 OPENSSL_EXPORT int X509_TRUST_get_by_id(int id);
 OPENSSL_EXPORT int X509_TRUST_add(int id, int flags,
                                   int (*ck)(X509_TRUST *, X509 *, int),
-                                  char *name, int arg1, void *arg2);
+                                  const char *name, int arg1, void *arg2);
 OPENSSL_EXPORT void X509_TRUST_cleanup(void);
 OPENSSL_EXPORT int X509_TRUST_get_flags(const X509_TRUST *xp);
 OPENSSL_EXPORT char *X509_TRUST_get0_name(const X509_TRUST *xp);