Adds a hidden drake_vendor namespace for ODR safety

--- picosha2.h
+++ picosha2.h
@@ -36,6 +36,7 @@
 #include <sstream>
 #include <vector>
 #include <fstream>
+inline namespace drake_vendor __attribute__ ((visibility ("hidden"))) {
 namespace picosha2 {
 typedef unsigned long word_t;
 typedef unsigned char byte_t;
@@ -374,4 +375,5 @@
 
 }
 }// namespace picosha2
+}// namespace drake_vendor
 #endif  // PICOSHA2_H
