Remove explicit FORTIFY_SOURCE flag

Local installation of the library complains of redefinition of this
flag, so remove the explicit definition. The default should be 3.

BUG=b:296063204
TEST=check that:
```
cd ~/trumk/src/platform/touchpad-tests
sudo make setup-in-place
touchtests
```
passes.

Change-Id: I9b94a90858d2be04a819ddcedd2143b819bb1fdb
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/libevdev/+/4957255
Reviewed-by: Denis Brockus <[email protected]>
Commit-Queue: Sean O'Brien <[email protected]>
Tested-by: Sean O'Brien <[email protected]>
diff --git a/common.mk b/common.mk
index a3b3e98..d7fb890 100644
--- a/common.mk
+++ b/common.mk
@@ -1,4 +1,4 @@
-# Copyright (c) 2012 The Chromium OS Authors. All rights reserved.
+# Copyright 2012 The ChromiumOS Authors
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 #
@@ -282,7 +282,6 @@
 COMMON_CFLAGS := -Wall -Werror -fno-strict-aliasing -O1 -Wformat=2
 CXXFLAGS += $(COMMON_CFLAGS) $(COMMON_CFLAGS-$(CXXDRIVER))
 CFLAGS += $(COMMON_CFLAGS) $(COMMON_CFLAGS-$(CDRIVER))
-CPPFLAGS += -D_FORTIFY_SOURCE=3
 
 
 ifeq ($(MODE),opt)