release: Update package.xml version to 2.6.4

package.xml

 <?xml version="1.0"?>
 <package format="3">
   <name>pinocchio</name>
-  <version>2.6.3</version>
+  <version>2.6.4</version>
   <description>A fast and flexible implementation of Rigid Body Dynamics algorithms and their analytical derivatives.</description>
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