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

third_party/libxml/libxml.gyp

Index: third_party/libxml/libxml.gyp
diff --git a/third_party/libxml/libxml.gyp b/third_party/libxml/libxml.gyp
index dc4c501ef5f9588873eabe2fdc871e686232baf6..d697bc41158ad9e196ecf4faddccd40d059bb8fe 100644
--- a/third_party/libxml/libxml.gyp
+++ b/third_party/libxml/libxml.gyp
@@ -131,7 +131,6 @@
             'src/include/libxml/xpointer.h',
             'src/include/win32config.h',
             'src/include/wsockcompat.h',
-            'src/acconfig.h',
             'src/buf.c',
             'src/buf.h',
             'src/c14n.c',
@@ -141,6 +140,7 @@
             'src/dict.c',
             'src/DOCBparser.c',
             'src/elfgcchack.h',
+            'src/enc.h',
             'src/encoding.c',
             'src/entities.c',
             'src/error.c',
@@ -155,10 +155,12 @@
             'src/parserInternals.c',
             'src/pattern.c',
             'src/relaxng.c',
+            'src/save.h',
             'src/SAX.c',
             'src/SAX2.c',
             'src/schematron.c',
             'src/threads.c',
+            'src/timsort.h',
             'src/tree.c',
             #'src/trio.c',
             #'src/trio.h',
@@ -185,6 +187,8 @@
             'src/xmlwriter.c',
             'src/xpath.c',
             'src/xpointer.c',
+            #'src/xzlib.c',
+            'src/xzlib.h',
             'win32/config.h',
             'win32/include/libxml/xmlversion.h',
           ],