[Sanitizers] Fix sanitizer_linux_libcdep.cc compilation on Solaris

Both LLVM 8.0.0 and current trunk fail to compile on Solaris 11/x86 with
GCC 8.1.0:

  /vol/llvm/src/llvm/dist/projects/compiler-rt/lib/sanitizer_common/sanitizer_linux_libcdep.cc: In function ‘void __sanitizer::ReExec()’:
  /vol/llvm/src/llvm/dist/projects/compiler-rt/lib/sanitizer_common/sanitizer_linux_libcdep.cc:831:14: error: ‘getexecname’ was not declared in this scope
     pathname = getexecname();
                ^~~~~~~~~~~
  /vol/llvm/src/llvm/dist/projects/compiler-rt/lib/sanitizer_common/sanitizer_linux_libcdep.cc:831:14: note: suggested alternative: ‘gethostname’
     pathname = getexecname();
                ^~~~~~~~~~~
                gethostname

This is easily fixed by including <stdlib.h> which declares that function.
With that patch, compilation continues.

Differential Revision: https://reviews.llvm.org/D60044

llvm-svn: 357751