netipmid: Add -flto to CXXFLAGS

Following the example of 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: Iad0c9ccf5c0409321da4aaef21882eb7171e14b0
Signed-off-by: Vernon Mauery <vernon.mauery@linux.intel.com>
diff --git a/Makefile.am b/Makefile.am
index 1d1a370..fb419df 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -70,6 +70,7 @@
 	-export-dynamic
 
 netipmid_CXXFLAGS = \
+	-flto \
 	$(SYSTEMD_CFLAGS) \
 	$(libmapper_CFLAGS) \
 	$(PHOSPHOR_LOGGING_CFLAGS) \