C89 compliance.

On CentOS 6.4, `python3.4 setup.py build` gives:
```
gcc -pthread -Werror=declaration-after-statement -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes -fPIC -I/usr/local/include/python3.4m -c src/kerberosgss.c -o build/temp.linux-x86_64-3.4/src/kerberosgss.o
src/kerberosgss.c: In function ‘authenticate_gss_client_step’:
src/kerberosgss.c:268: error: ISO C90 forbids mixed declarations and code
src/kerberosgss.c: In function ‘authenticate_gss_server_init’:
src/kerberosgss.c:449: error: ISO C90 forbids mixed declarations and code
error: command 'gcc' failed with exit status 1
```
Fixing that yields similar errors in `base64.c`.

`gcc --version` is `gcc (GCC) 4.4.7 20120313 (Red Hat 4.4.7-4)`.

The change I'm proposing makes the code C89 compliant, and therefore C90 compliant. It fixes the compile error and complies with PEP7, which states:

> Use ANSI/ISO standard C (the 1989 version of the standard). This means (amongst many other things) that all declarations must be at the top of a block.