bugfix: build: add -lstdc++fs to Makefile_am

Add -lstdc++fs to Makefile_am as it's now used by the file_handler
module.

Change-Id: Id96f7d8d4b74870a7d65af3836b57029a35cda2c
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/Makefile.am b/Makefile.am
index 75c84cd..6265ed8 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -15,6 +15,7 @@
 libfirmwareblob_la_LDFLAGS = \
 	$(SDBUSPLUS_LIBS) \
 	$(PHOSPHOR_LOGGING_LIBS) \
+	-lstdc++fs \
 	-version-info 0:0:0 -shared
 libfirmwareblob_la_CXXFLAGS = \
 	$(SDBUSPLUS_CFLAGS) \