ldlinux: Tag __syslinux_init() as __constructor

ldlinux: Tag __syslinux_init() as __constructor

We don't need to have control over exactly when __syslinux_init() is
executed, it just needs to be before we call syslinux_get_adv(). Let's
apply the __constructor tag to __syslinux_init() so that it is
executed before we enter ldlinux.c32's main(). This partially reverts
commit 487b67a9ce ("ldlinux: Remove __constructor attribute") because
we can now have __constructor functions in ldlinux.

By not calling __syslinux_init() directly we also avoid the following
build warning,

ldlinux.c: In function ‘main’:
ldlinux.c:131:2: warning: implicit declaration of function ‘__syslinux_init’

Signed-off-by: Matt Fleming <matt.fleming@intel.com>