Add linux pacman add_extsources for automake (#871)

packages/a/automake/xmake.lua

@@ -13,7 +13,7 @@ package("automake")
     add_versions("1.9.5", "68712753fcb756f3707b7da554917afb348450eb8530cae3b623a067078596fd")
 
     if is_host("linux") then
-        add_extsources("apt::automake")
+        add_extsources("apt::automake", "pacman::automake")
     end
 
     add_deps("autoconf")