oox: use proper integer type when calling PK11_DigestFinal

Change-Id: I5ff79c55446bd9f3e350fb128793ffef51be843f
configure: build oox with NSS backend by default

Because NSS libraries are dynamic and OpenSSL static, using NSS saves
1.5 MB in the oox library [even though it's not as 1337 apparently]:

-rwxrwxr-x. 1 ms ms  8889575  2. Nov 13:45 libooxlo.so.nss
-rwxrwxr-x. 1 ms ms  7773576  2. Nov 13:45 libooxlo.so.nss.stripped
-rwxrwxr-x. 1 ms ms 10340276  2. Nov 13:37 libooxlo.so.openssl
-rwxrwxr-x. 1 ms ms  9042216  2. Nov 13:37 libooxlo.so.openssl.stripped

Change-Id: I387496ae364acb1286d753d52f04924631136750