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/crypto/x509/x509_trs.c b/crypto/x509/x509_trs.c
index 13e5eca..71cf71d 100644
--- a/crypto/x509/x509_trs.c
+++ b/crypto/x509/x509_trs.c
@@ -168,7 +168,7 @@
 }
 
 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) {
   int idx;
   X509_TRUST *trtmp;
   char *name_dup;