build: Add -flto to CXXFLAGS

Add -flto to CXXFLAGS to allow build optimization.

Change-Id: I96e05b258411a830b9cdf9776bfd23333eac3a5d
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/Makefile.am b/Makefile.am
index 40ea490..f63be84 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -8,4 +8,5 @@
 	-lstdc++fs \
 	-version-info 0:0:0 -shared
 
-libethstatscmd_la_CXXFLAGS =
+libethstatscmd_la_CXXFLAGS = \
+	-flto