Move the X509_NAME typedef into x509.h.

X509_NAME is one of the symbols that collide with wincrypt.h. Move it to x509.h
so libraries which only use the pure-crypto portions of BoringSSL without X.509
needn't have to resolve the collision.

Change-Id: I057873498e58fe4a4cf264356f9a58d7a15397b7
Reviewed-on: https://boringssl-review.googlesource.com/2080
Reviewed-by: Adam Langley <agl@google.com>