build: Add -flto to CXXFLAGS

Add -flto to CXXFLAGS to allow build optimization.

Change-Id: Id80576cc9928de46cf85c0b39da0d54ff8fb3d40
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/Makefile.am b/Makefile.am
index ba5a539..84b0c56 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -17,4 +17,5 @@
 
 libsyscmds_la_CXXFLAGS = \
 	$(SDBUSPLUS_CFLAGS) \
-	$(PHOSPHOR_LOGGING_CFLAGS)
+	$(PHOSPHOR_LOGGING_CFLAGS) \
+	-flto