Add <stdlib.h> includes for clang++/libc++ on Ubuntu

Add <stdlib.h> includes for clang++/libc++ on Ubuntu

There are a number of stdlib.h uses in these files without including
stdlib.h:

 hash_collision_bench.cc: rand, RAND_MAX, srand
 manifest_parser_perftest.cc: system, exit
 ninja_test.cc: EXIT_SUCCESS, EXIT_FAILURE
 test.cc: getenv, mkdtemp, system

This works on a Ubuntu g++/libstdc++ build, as the <algorithm> header
pulls in stdlib.h, and on a OSX clang++/libc++ build the <map> and
<string> headers pull in stdlib.h. But a Ubuntu clang++/libc++ build
does not pull in stdlib.h with any of these other headers.

 $ apt-get install clang-3.6 libc++-dev
 $ CXX=clang++-3.6 CFLAGS=-stdlib=libc++ LDFLAGS=-stdlib=libc++ \
   ./configure.py
 $ ninja ninja_test hash_collision_bench manifest_parser_perftest

This was originally discovered using the host toolchain provided with
Android, but the Ubuntu version is much easier to reproduce.