Move declaration for alias.c to alias.h

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/pager.c b/pager.c
index 92b23e6..1f4688f 100644
--- a/pager.c
+++ b/pager.c
@@ -2,6 +2,7 @@
 #include "config.h"
 #include "run-command.h"
 #include "sigchain.h"
+#include "alias.h"
 
 #ifndef DEFAULT_PAGER
 #define DEFAULT_PAGER "less"