Fix namespacing of err_save_state_st.

In I1893343fee1b43b3e31e9d66a244475ffc4a35f1, this struct was moved to
the bssl namespace.

Turns out it does not work to define a struct this way; it causes deltas
in the symbol names when symbol prefixing is turned on:

$ for f in libcrypto.a CMakeFiles/crypto_test.dir/**/*.o; do nm -a "$f" | grep ERR_restore_state; done
0000000000001a08 T __ZN4bssl14brlogenshfegle17ERR_restore_stateEPKNS_17err_save_state_stE
                 U __ZN4bssl14brlogenshfegle17ERR_restore_stateEPKNS0_17err_save_state_stE
$ for f in libcrypto.a CMakeFiles/crypto_test.dir/**/*.o; do nm --demangle -a "$f" | grep ERR_restore_state; done
0000000000001a08 T bssl::brlogenshfegle::ERR_restore_state(bssl::err_save_state_st const*)
                 U bssl::brlogenshfegle::ERR_restore_state(bssl::brlogenshfegle::err_save_state_st const*)

So it looks like the bssl:: hack can currently only be used for variable
and function definitions, and most definitely not for types.

Note that this issue only happens with Clang, not with GCC:
https://godbolt.org/z/nsq5K95dc <-> https://godbolt.org/z/fhfMn5Td6

Bug: 42220000
Change-Id: Ic1396212ce1baed6b53a4ed92c089a1d145bd480
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/86067
Auto-Submit: Rudolf Polzer <rpolzer@google.com>
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>

crypto/err/err.cc

diff --git a/crypto/err/err.cc b/crypto/err/err.cc
index 491070c..3c51ae4 100644
--- a/crypto/err/err.cc
+++ b/crypto/err/err.cc
@@ -747,11 +747,15 @@
 
 void ERR_load_RAND_strings(void) {}
 
-struct bssl::err_save_state_st {
+BSSL_NAMESPACE_BEGIN
+
+struct err_save_state_st {
   struct err_error_st *errors;
   size_t num_errors;
 };
 
+BSSL_NAMESPACE_END
+
 void bssl::ERR_SAVE_STATE_free(ERR_SAVE_STATE *state) {
   if (state == nullptr) {
     return;