Fix all implicit declarations

AC_HEADER_TIME is only needed if you need to include time.h and
sys/time.h at the same time.

The truth is this codebase is an unholy mess but at least this fixes
the warnings.

module.c

@@ -13,9 +13,6 @@
 #ifdef HAVE_SYS_WAIT_H
 #include <sys/wait.h>
 #endif
-#ifdef HAVE_SYS_TIME_H
-#include <sys/time.h>
-#endif
 #ifdef HAVE_SYS_RESOURCE_H
 #include <sys/resource.h>
 #endif
@@ -42,7 +39,6 @@ extern Display *dpy;
 extern void add_fd_to_set(int);
 extern void remove_fd_from_set(int);
 
-extern void screentoback();
 extern void raiselowerclient(Client *, int);
 extern void wberror(Scrn *, char *);
 extern void reparent(Client *);