init: fix type of 2nd argument passed to prctl

prctl(PR_SET_SECUREBITS, ...) expects an unsigned long as its 2nd argument.
Passing in a int64_t happens to work with a 64-bit kernel, but does not
work with a 32-bit kernel.

Bug: 63680332
Test: boot 32-bit kernel; verify services with capabilities can successfully
      set those capabilties
Change-Id: I60250d107a77b54b2e9fe3419b4480b921c7e2f8
Signed-off-by: Ben Fennema <fennema@google.com>