Clang16 will not suppert implicit int and implicit function declarations.
This patch makes the source code ready for clang16. 
See also: https://bugs.gentoo.org/870412

Bug: https://bugs.gentoo.org/874102

This patch is already merged upstream, see: https://github.com/folkertvanheusden/multitail/pull/10

Signed-off-by: Pascal Jäger <pascal.jaeger@leimstift.de>

--- a/misc.c
+++ b/misc.c
@@ -477,7 +477,7 @@ void heartbeat(void)
 	mydoupdate();
 }
 
-void do_check_for_mail()
+void do_check_for_mail(dtime_t time)
 {
 	if (check_for_mail > 0 && mail_spool_file != NULL)
 	{
diff --git a/misc.h b/misc.h
index febf11a..5566519 100644
--- a/misc.h
+++ b/misc.h
@@ -1,5 +1,7 @@
+#import "mt.h"
+
 void info(void);
 void statistics_menu(void);
 void heartbeat(void);
-void do_check_for_mail();
+void do_check_for_mail(dtime_t time);
 void store_statistics(proginfo *cur, dtime_t now);