irc-login.c, login.c: add missing include of "string.h"

irc-login.c, login.c: add missing include of "string.h"

This fixes the following warnings with GCC 4.4.5 on Linux:

 irc-login.c: In function ‘IRC_PASS’:
 irc-login.c:92: warning: implicit declaration of function ‘strlen’
 irc-login.c:92: warning: incompatible implicit declaration of built-in function ‘strlen’
 irc-login.c:113: warning: incompatible implicit declaration of built-in function ‘strlen’
 irc-login.c:129: warning: implicit declaration of function ‘strchr’
 irc-login.c:129: warning: incompatible implicit declaration of built-in function ‘strchr’
 irc-login.c:133: warning: implicit declaration of function ‘strcmp’
 irc-login.c: In function ‘IRC_SERVICE’:
 irc-login.c:556: warning: incompatible implicit declaration of built-in function ‘strchr’
 login.c: In function ‘Login_User’:
 login.c:131: warning: implicit declaration of function ‘strcmp’