exception: move InvalidEnumString functions to library

Change-Id: Ic5020eddb48d22041a51a39bae2090f379aeaeb6
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/Makefile.am b/Makefile.am
index 175355c..4bed320 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -22,7 +22,8 @@
 	sdbusplus/utility/type_traits.hpp \
 	sdbusplus/vtable.hpp
 
-LIBSDBUSPLUS_FILES =
+LIBSDBUSPLUS_FILES = \
+	sdbusplus/exception.cpp
 
 if WANT_TRANSACTION
 LIBSDBUPLUS_FILES_TRANSACTION = \