zstd: change pkgconfig file name to libzstd.pc (#1077)

packages/z/zstd/xmake.lua

@@ -12,6 +12,7 @@ package("zstd")
     on_install(function (package)
         io.writefile("xmake.lua", [[
             add_rules("mode.debug", "mode.release", "asm")
+            add_rules("utils.install.pkgconfig_importfiles", {filename = "libzstd.pc"})
             target("zstd")
                 set_kind("$(kind)")
                 add_files("lib/common/*.c")