Use xmllint built from source

Instead of relying on whatever version of xmllint is on the host system,
build and use the version in external/libxml2.

Test: diff build_aosp-marlin.ninja, expected changes.
Test: m $(xmllint targets in build_aosp-marlin.ninja)
Test: introduce xml error, build fails
Change-Id: I39579f06db3777e3b5c8dda7c7541c25a35887b2
diff --git a/core/config.mk b/core/config.mk
index 4942be7..295644c 100644
--- a/core/config.mk
+++ b/core/config.mk
@@ -580,6 +580,7 @@
 SOONG_JAVAC_WRAPPER := $(SOONG_HOST_OUT_EXECUTABLES)/soong_javac_wrapper
 SOONG_ZIP := $(SOONG_HOST_OUT_EXECUTABLES)/soong_zip
 MERGE_ZIPS := $(SOONG_HOST_OUT_EXECUTABLES)/merge_zips
+XMLLINT := $(SOONG_HOST_OUT_EXECUTABLES)/xmllint
 ZIP2ZIP := $(SOONG_HOST_OUT_EXECUTABLES)/zip2zip
 ZIPTIME := $(prebuilt_build_tools_bin)/ziptime