Linux: Change the default for seccomp soft-fail.

Make seccomp hard-fail by default, and soft-fail when USE_seccomp is
"no".

This simplifies building on Linux because most folks no longer have to
pass USE_seccomp=yes when building.

Bug: 37162444
Test: Builds with USE_seccomp=yes, with USE_seccomp=no, without.
Test: With USE_seccomp=yes and without produces the same binary.
Change-Id: I3821f5b7d2db78e334107263fed0a591695540a3
diff --git a/Makefile b/Makefile
index 48d9f9d..9b06a94 100644
--- a/Makefile
+++ b/Makefile
@@ -12,7 +12,8 @@
 ifneq ($(HAVE_SECUREBITS_H),no)
 CPPFLAGS += -DHAVE_SECUREBITS_H
 endif
-ifneq ($(USE_seccomp),yes)
+
+ifeq ($(USE_seccomp),no)
 CPPFLAGS += -DUSE_SECCOMP_SOFTFAIL
 endif