fix sdist for building from sources via pip install

```siphashc``` fails to build from sources because header files are missing.
This PR sets the include directory on for the C-Extension so that it can be build from source.

```
$ pip install siphashc==0.6
Downloading/unpacking siphashc==0.6
  Downloading siphashc-0.6.tar.gz
  Running setup.py (path:/Users/god/Envs/heaven/build/siphashc/setup.py) egg_info for package siphashc

Requirement already satisfied (use --upgrade to upgrade): setuptools in /Users/god/Envs/heaven/lib/python2.7/site-packages (from siphashc==0.6)
Installing collected packages: siphashc
  Running setup.py install for siphashc
    building 'siphashc' extension
    clang -fno-strict-aliasing -fno-common -dynamic -I/usr/local/include -I/usr/local/opt/sqlite/include -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes -Wno-error=unused-command-line-argument-hard-error-in-future -I/usr/local/Cellar/python/2.7.8/Frameworks/Python.framework/Versions/2.7/include/python2.7 -c src/siphash/siphash.c -o build/temp.macosx-10.9-x86_64-2.7/src/siphash/siphash.o
    src/siphash/siphash.c:1:10: fatal error: 'siphash.h' file not found
    #include "siphash.h"
             ^
    1 error generated.
    error: command 'clang' failed with exit status 1
    Complete output from command /Users/god/Envs/heaven/bin/python2.7 -c "import setuptools, tokenize;__file__='/Users/god/Envs/heaven/build/siphashc/setup.py';exec(compile(getattr(tokenize, 'open', open)(__file__).read().replace('\r\n', '\n'), __file__, 'exec'))" install --record /var/folders/l4/lzmbbdfj1t7dmypcqf_t61g80000gn/T/pip-3e0xQu-record/install-record.txt --single-version-externally-managed --compile --install-headers /Users/god/Envs/heaven/bin/../include/site/python2.7:
    running install

running build

running build_py

creating build

creating build/lib.macosx-10.9-x86_64-2.7

creating build/lib.macosx-10.9-x86_64-2.7/tests

copying tests/__init__.py -> build/lib.macosx-10.9-x86_64-2.7/tests

copying tests/test_siphashc.py -> build/lib.macosx-10.9-x86_64-2.7/tests

running build_ext

building 'siphashc' extension

creating build/temp.macosx-10.9-x86_64-2.7

creating build/temp.macosx-10.9-x86_64-2.7/src

creating build/temp.macosx-10.9-x86_64-2.7/src/siphash

clang -fno-strict-aliasing -fno-common -dynamic -I/usr/local/include -I/usr/local/opt/sqlite/include -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes -Wno-error=unused-command-line-argument-hard-error-in-future -I/usr/local/Cellar/python/2.7.8/Frameworks/Python.framework/Versions/2.7/include/python2.7 -c src/siphash/siphash.c -o build/temp.macosx-10.9-x86_64-2.7/src/siphash/siphash.o

src/siphash/siphash.c:1:10: fatal error: 'siphash.h' file not found

#include "siphash.h"

         ^

1 error generated.

error: command 'clang' failed with exit status 1

----------------------------------------
Cleaning up...
Command /Users/god/Envs/heaven/bin/python2.7 -c "import setuptools, tokenize;__file__='/Users/god/Envs/heaven/build/siphashc/setup.py';exec(compile(getattr(tokenize, 'open', open)(__file__).read().replace('\r\n', '\n'), __file__, 'exec'))" install --record /var/folders/l4/lzmbbdfj1t7dmypcqf_t61g80000gn/T/pip-3e0xQu-record/install-record.txt --single-version-externally-managed --compile --install-headers /Users/god/Envs/heaven/bin/../include/site/python2.7 failed with error code 1 in /Users/god/Envs/heaven/build/siphashc
Storing debug log for failure in /Users/god/.pip/pip.log
```