Add linux pacman add_extsources for libxt (#869)

packages/l/libxt/xmake.lua

@@ -7,7 +7,7 @@ package("libxt")
     add_versions("1.2.1", "6da1bfa9dd0ed87430a5ce95b129485086394df308998ebe34d98e378e3dfb33")
 
     if is_plat("linux") then
-        add_extsources("apt::libxt-dev")
+        add_extsources("apt::libxt-dev", "pacman::libxt")
     end
 
     add_deps("libx11", "libsm")