move getEnv methods into env namespace

Move the getEnv(...) methods into the env namespace,
for consistency.

Change-Id: I4055d9456c17f8b20071cdee1b8e531c10fb0c7e
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/env.hpp b/env.hpp
index e14dd17..50a9fdb 100644
--- a/env.hpp
+++ b/env.hpp
@@ -4,6 +4,8 @@
 
 #include "sensorset.hpp"
 
+namespace env {
+
 /** @brief Reads an environment variable
  *
  *  Reads <prefix>_<sensor.first><sensor.second>
@@ -40,3 +42,5 @@
 std::string getIndirectID(
     std::string path,
     const SensorSet::key_type& sensor);
+
+}  // namespace env