bugfix: build: add -lstdc++fs to Makefile_am
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 <36c53fa43f47f3fd67cb2fc38031827244d18a1f@google.com>

openbmc/phosphor-ipmi-flash