Rename safe_stack.h to safestack.h.

This empty header file exists only to make older code compile. But I
named it incorrectly! Upstream doesn't have the underscore in the name.

Change-Id: I96654b7e17d84a5f2810e6eb20fe7bfb22f855fd
Reviewed-on: https://boringssl-review.googlesource.com/3540
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/include/openssl/safe_stack.h b/include/openssl/safestack.h
similarity index 100%
rename from include/openssl/safe_stack.h
rename to include/openssl/safestack.h