sha1: win32: fix compilation due to unknown type

In commit bbf034ab (hash: move `git_hash_prov` into Win32 backend,
2019-02-22), the `git_hash_prov`'s structure name has been removed in
favour of its typedef'ed name. But as we have no CI that compiles with
the WinHTTPS hashing backend right now, it wasn't noticed that the
implementation that uses this struct wasn't changed correctly.

Fix the struct type to make it compile again.

src/hash/sha1/win32.c

@@ -23,7 +23,7 @@
 /* BCRYPT_HASH_REUSEABLE_FLAGS */
 #define GIT_HASH_CNG_HASH_REUSABLE      0x00000020
 
-static struct git_hash_prov hash_prov = {0};
+static git_hash_prov hash_prov = {0};
 
 /* Hash initialization */