Fix/workaround build breakage caused by PAM import

struct pam_conv takes a void * for the appdata_ptr  but is being passed
a const char * - explicitly cast away the const

usr.sbin/ppp/auth.c

@@ -140,7 +140,7 @@ auth_CheckPasswd(const char *name, const char *data, const char *key)
 #ifdef _OPENPAM
       &openpam_nullconv, NULL
 #else
-      &pam_conv, key
+      &pam_conv, (char *)key
 #endif
     };