user_channel: strlen isn't constexpr

Change-Id: Id08b03dc487a1c772fa39459cc4e6e3138675910
Signed-off-by: Peter Foley <pefoley@google.com>
diff --git a/user_channel/channel_mgmt.cpp b/user_channel/channel_mgmt.cpp
index 2d492e0..50505aa 100644
--- a/user_channel/channel_mgmt.cpp
+++ b/user_channel/channel_mgmt.cpp
@@ -181,7 +181,7 @@
 
 std::string ChannelConfig::getChannelNameFromPath(const std::string& path)
 {
-    constexpr size_t length = strlen(networkIntfObjectBasePath);
+    const size_t length = strlen(networkIntfObjectBasePath);
     if (((length + 1) >= path.size()) ||
         path.compare(0, length, networkIntfObjectBasePath))
     {