init: Fix a compiler warning

Fix the following compiler warning:

system/core/init/init.cpp:754:57: warning: ISO C++ requires field designators to be specified in declaration order; field '' will be initialized after field 'sa_flags' [-Wreorder-init-list]
    const struct sigaction act { .sa_handler = SIG_DFL, .sa_flags = SA_NOCLDSTOP };
                                                        ^~~~~~~~~~~~~~~~~~~~~~~~
system/core/init/init.cpp:754:34: note: previous initialization for field '' is here
    const struct sigaction act { .sa_handler = SIG_DFL, .sa_flags = SA_NOCLDSTOP };
                                 ^~~~~~~~~~~~~~~~~~~~~

Change-Id: I29e2d51dfdff85212a33eebfd51b241268cdfe9a
Signed-off-by: Bart Van Assche <bvanassche@google.com>