fix warnings and compile error in Apache2.4


## environment

 * Linux Mint 14 kernel-3.5.0-31-generic amd64
 * GCC 4.7.2
 * Apache 2.4.4

In my environment, the building mod_access_token fails.

```
$ autoconf
$ ./configure --with-apxs=/usr/local/apache2/bin/apxs
$ make
/usr/local/apache2/bin/apxs -c   mod_access_token.c 
/usr/share/apr-1.0/build/libtool --silent --mode=compile x86_64-linux-gnu-gcc -std=gnu99 -prefer-pic   -D_REENTRANT -D_GNU_SOURCE -pthread -I/usr/local/apache2/include  -I/usr/include/apr-1.0   -I/usr/include/apr-1.0 -I/usr/include  -c -o mod_access_token.lo mod_access_token.c && touch mod_access_token.slo
mod_access_token.c: In function ‘hmac_sha1’:
mod_access_token.c:73:5: warning: implicit declaration of function ‘apr_base64_encode_len’ [-Wimplicit-function-declaration]
mod_access_token.c:75:5: warning: implicit declaration of function ‘apr_base64_encode_binary’ [-Wimplicit-function-declaration]
mod_access_token.c: In function ‘access_token_parse_args’:
mod_access_token.c:111:13: error: too few arguments to function ‘ap_unescape_url_keep2f’
In file included from mod_access_token.c:2:0:
/usr/local/apache2/include/httpd.h:1569:17: note: declared here
mod_access_token.c: In function ‘access_token_register_hooks’:
mod_access_token.c:175:5: warning: implicit declaration of function ‘ap_hook_access_checker’ [-Wimplicit-function-declaration]
apxs:Error: Command failed with rc=65536
.
make: *** [all] Error 1
$
```

This patch will resolve above warnings and errors.