Add -flto to CXXFLAGS

Per openbmc/openbmc#3364, adding -flto to CXX flags
in order to reduce overall library and binary sizes.

Ref: https://gcc.gnu.org/wiki/LinkTimeOptimization

Change-Id: I3ee31a15fc0ded4308aa9d00a11b41bc62eb9799
Signed-off-by: Andrew Geissler <geissonator@yahoo.com>
diff --git a/control/Makefile.am b/control/Makefile.am
index 09fb1a2..01ee9c9 100644
--- a/control/Makefile.am
+++ b/control/Makefile.am
@@ -24,7 +24,8 @@
 phosphor_fan_control_CXXFLAGS = \
 	$(SDBUSPLUS_CFLAGS) \
 	$(PHOSPHOR_LOGGING_CFLAGS) \
-	${PHOSPHOR_DBUS_INTERFACES_CFLAGS}
+	${PHOSPHOR_DBUS_INTERFACES_CFLAGS} \
+	-flto
 
 BUILT_SOURCES = fan_zone_defs.cpp