Suppress chip data XML validation output

For some reason xmllint will output to stderr even when a file
validates. This clutters build output. So this change suppresses the
output and only prints on error.

Signed-off-by: Zane Shelley <zshelle@us.ibm.com>
Change-Id: I0e693f464f5efa10182d97635e0bba83b11b1330
diff --git a/xml/build_chip_data_binary b/xml/build_chip_data_binary
index 2844fd5..ab021b5 100755
--- a/xml/build_chip_data_binary
+++ b/xml/build_chip_data_binary
@@ -162,8 +162,8 @@
             my $path = "$dir/$file";
 
             # Ensure the XML is well-formed and validates against the schema.
-            system( "xmllint --noout --schema $RealBin/$type.xsd $path" );
-            die "RAS XML validation failed on $file" if ( 0 != $? );
+            my $out = `xmllint --noout --schema $RealBin/$type.xsd $path 2>&1`;
+            die "$out\nRAS XML validation failed on $file" if ( 0 != $? );
 
             # Import the XML.
             my $xml = XMLin( $path, KeyAttr => {}, ForceArray => 1 );