[libxml] remove Makefile from sources

This change removes the Makefile.am item from the sources list, enabling
the library to build correctly.

Test: added libxml2 as local dep, built fuchsia
Change-Id: Ib8f0d793b14c7d2b0f7adde25600176907ff9680
diff --git a/BUILD.gn b/BUILD.gn
index 8a4b59b..44bbac1 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -82,7 +82,6 @@
     "include/libxml/DOCBparser.h",
     "include/libxml/HTMLparser.h",
     "include/libxml/HTMLtree.h",
-    "include/libxml/Makefile.am",
     "include/libxml/SAX.h",
     "include/libxml/SAX2.h",
     "include/libxml/c14n.h",