Add missing signal.h header:

/havana/work/llvm/projects/compiler-rt/test/asan/TestCases/Linux/swapcontext_annotation.cc:158:20: error: variable has incomplete type 'struct sigaction'
  struct sigaction act = {};
                   ^
/havana/work/llvm/projects/compiler-rt/test/asan/TestCases/Linux/swapcontext_annotation.cc:158:10: note: forward declaration of 'sigaction'
  struct sigaction act = {};
         ^
/havana/work/llvm/projects/compiler-rt/test/asan/TestCases/Linux/swapcontext_annotation.cc:160:17: error: use of undeclared identifier 'SIGPROF'
  if (sigaction(SIGPROF, &act, 0)) {
                ^
2 errors generated.

llvm-svn: 319532