tools/usbip: fixes build with musl libc toolchain

Indeed musl doesn't define old SIGCLD signal name but only new one SIGCHLD.
SIGCHLD is the new POSIX name for that signal so it doesn't change
anything on other libcs.

This fixes this kind of build error:

usbipd.c: In function ‘set_signal’:
usbipd.c:459:12: error: 'SIGCLD' undeclared (first use in this function)
  sigaction(SIGCLD, &act, NULL);
            ^~~~~~
usbipd.c:459:12: note: each undeclared identifier is reported only once
	for each function it appears in
Makefile:407: recipe for target 'usbipd.o' failed
make[3]: *** [usbipd.o] Error 1

Signed-off-by: Julien BOIBESSOT <julien.boibessot@armadeus.com>
Acked-by: Shuah Khan <shuahkh@osg.samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>