Fix broken fuzzer compilation

fuzz_group_decrypt.cpp got broken by 653790e; fix it up

fuzzers/README.rst

@@ -45,7 +45,7 @@ Usage notes:
 
    .. code::
 
-      ./afl-fuzz -i- -o existing_output_dir [...etc...]
+       afl-fuzz -i- -o existing_output_dir [...etc...]
 
 8. If it shows failures, pipe the failure case into
    ``./build/fuzzers/debug_<fuzzing_tool>``, fix, and repeat.

fuzzers/fuzz_group_decrypt.cpp

@@ -54,6 +54,8 @@ int main(int argc, const char *argv[]) {
 
     uint8_t plaintext[max_length];
 
+    uint32_t ratchet_index;
+
     size_t length = check_error(
         olm_inbound_group_session_last_error,
         session,
@@ -61,7 +63,7 @@ int main(int argc, const char *argv[]) {
         olm_group_decrypt(
             session,
             message_buffer, message_length,
-            plaintext, max_length
+            plaintext, max_length, &ratchet_index
         )
     );