build: fix -Wmissing-prototype

This helps make the author's original intent clear.  Tested by building
and running the unit tests with clang.

Change-Id: If0231ec5634adad0b61e7fcff8ec1e5bcbbb92c1
Signed-off-by: Brad Bishop <bradbish@qti.qualcomm.com>
diff --git a/src/test/interfaces_added.cpp b/src/test/interfaces_added.cpp
index 7117611..c60298e 100644
--- a/src/test/interfaces_added.cpp
+++ b/src/test/interfaces_added.cpp
@@ -15,8 +15,8 @@
 
 // This is the data structure that comes in via the InterfacesAdded
 // signal
-InterfacesAdded createInterfacesAdded(const std::string& interface,
-                                      const std::string& property)
+static InterfacesAdded createInterfacesAdded(const std::string& interface,
+                                             const std::string& property)
 {
     std::vector<Association> associations = {
         {"inventory", "error",