Add linux pacman add_extsources for gperf (#880)

packages/g/gperf/xmake.lua

@@ -9,7 +9,7 @@ package("gperf")
     add_versions("3.1", "588546b945bba4b70b6a3a616e80b4ab466e3f33024a352fc2198112cdbb3ae2")
 
     if is_plat("linux") then
-        add_extsources("apt::gperf")
+        add_extsources("apt::gperf", "pacman::gperf")
     end
 
     on_install("macosx", "linux", function (package)