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/definitions.mk b/core/definitions.mk
index 2f3f356..f6f5840 100644
--- a/core/definitions.mk
+++ b/core/definitions.mk
@@ -2686,9 +2686,9 @@
 # $(1): source file
 # $(2): destination file, must end with .xml.
 define copy-xml-file-checked
-$(2): $(1)
+$(2): $(1) $(XMLLINT)
 	@echo "Copy xml: $$@"
-	$(hide) xmllint $$< >/dev/null  # Don't print the xml file to stdout.
+	$(hide) $(XMLLINT) $$< >/dev/null  # Don't print the xml file to stdout.
 	$$(copy-file-to-target)
 endef