Add linux pacman add_extsources for libxtst (#868)

packages/l/libxtst/xmake.lua

@@ -8,7 +8,7 @@ package("libxtst")
     add_versions("1.2.3", "4655498a1b8e844e3d6f21f3b2c4e2b571effb5fd83199d428a6ba7ea4bf5204")
 
     if is_plat("linux") then
-        add_extsources("apt::libxtst-dev")
+        add_extsources("apt::libxtst-dev", "pacman::libxtst")
     end
 
     if is_plat("macosx", "linux") then