Add linux pacman add_extsources for libtool (#895)

packages/l/libtool/xmake.lua

@@ -11,7 +11,7 @@ package("libtool")
     add_versions("2.4.5", "509cb49c7de14ce7eaf88993cf09fd4071882699dfd874c2e95b31ab107d6987")
 
     if is_host("linux") then
-        add_extsources("apt::libtool")
+        add_extsources("apt::libtool", "pacman::libtool")
     end
 
     add_deps("autoconf")