[TestSendSignal] Include signal.h instead of sys/signal.h in the test case.

Summary:
Android API-9 does not have sys/signal.h. However, all sys/signal.h
has when present is an include of signal.h.

Test Plan: dotest.py -p TestSendSignal on linux and Android.

Reviewers: chaoren

Reviewed By: chaoren

Subscribers: tberghammer, lldb-commits

Differential Revision: http://reviews.llvm.org/D9779

llvm-svn: 237381