util/sha1: add non-typedef name for the SHA1_CTX struct

util/sha1: add non-typedef name for the SHA1_CTX struct

Using typedef(s) is not always the answer and makes it harder for people
to do clever (or one might call nasty) things with the code.

Add a struct name which we will use with follow-up commit.

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Grazvydas Ignotas <notasas@gmail.com>