Add linux pacman add_extsources for libxau (#892)

packages/l/libxau/xmake.lua

@@ -7,7 +7,7 @@ package("libxau")
     add_versions("1.0.9", "ccf8cbf0dbf676faa2ea0a6d64bcc3b6746064722b606c8c52917ed00dcb73ec")
 
     if is_plat("linux") then
-        add_extsources("apt::libxau-dev")
+        add_extsources("apt::libxau-dev", "pacman::libxau")
     end
 
     if is_plat("macosx", "linux") then