clatd: move stop_loop() into main.c

* New clang-tidy gives more warnings:
  .../external/android-clat/main.c:136:23: error: 'stop_loop' may not be asynchronous-safe; calling it from a signal handler may be dangerous [cert-sig30-c,-warnings-as-errors]
  if (signal(SIGTERM, stop_loop) == SIG_ERR) {
                      ^
  .../external/android-clat/main.c:136:7: note: signal handler registered here
  if (signal(SIGTERM, stop_loop) == SIG_ERR) {
      ^
  external/android-clat/clatd.h:38:1: note: handler function declared here
    void stop_loop();
    ^

Bug: 179304856
Test: builds, atest, TreeHugger
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I06bad709df2ddded97829833f5e86d72ac282fa3
3 files changed