[cru] Use hidden linker visibility

This prevents ODR violations in case downstream code also links to CRU.

--- include/common_robotics_utilities/cru_namespace.hpp
+++ include/common_robotics_utilities/cru_namespace.hpp
@@ -17,5 +17,5 @@
 
 #ifndef CRU_NAMESPACE_BEGIN
-# define CRU_NAMESPACE_BEGIN inline namespace v1 {
+# define CRU_NAMESPACE_BEGIN inline namespace drake_vendor __attribute__ ((visibility ("hidden"))) {
 #endif
 
