Update libxml gyp and BUILD.gn, was missing some .h files

third_party/libxml/BUILD.gn

Index: third_party/libxml/BUILD.gn
diff --git a/third_party/libxml/BUILD.gn b/third_party/libxml/BUILD.gn
index e6e10899dcdf809f49bfdfce6e9d8794c454d581..722a4a43cd2e71966801d4832d680f91117f024f 100644
--- a/third_party/libxml/BUILD.gn
+++ b/third_party/libxml/BUILD.gn
@@ -38,7 +38,6 @@ static_library("libxml") {
     "src/HTMLtree.c",
     "src/SAX.c",
     "src/SAX2.c",
-    "src/acconfig.h",
     "src/buf.c",
     "src/buf.h",
     "src/c14n.c",
@@ -47,6 +46,7 @@ static_library("libxml") {
     "src/debugXML.c",
     "src/dict.c",
     "src/elfgcchack.h",
+    "src/enc.h",
     "src/encoding.c",
     "src/entities.c",
     "src/error.c",
@@ -105,8 +105,10 @@ static_library("libxml") {
     "src/parserInternals.c",
     "src/pattern.c",
     "src/relaxng.c",
+    "src/save.h",
     "src/schematron.c",
     "src/threads.c",
+    "src/timsort.h",
     "src/tree.c",
 
     #"src/trio.c",
@@ -134,6 +136,8 @@ static_library("libxml") {
     "src/xmlwriter.c",
     "src/xpath.c",
     "src/xpointer.c",
+    #"src/xzlib.c",
+    "src/xzlib.h",
     "win32/config.h",
     "win32/include/libxml/xmlversion.h",
   ]