Add #include <unistd.h> to provide prototype for getopt()

[github at samba.org]

New comment by candlerb on Samba Github repository

https://github.com/samba-team/samba/pull/116#issuecomment-355042888
Comment:
Travis 225.1 to 225.7 inclusive are failing with 

~~~
Checking for macro gnutls_aead_cipher_init                                        : not found 
No gnutls support for AEAD encryption
/tmp/b9051/samba-xc/source4/dsdb/samdb/ldb_modules/wscript:43: error: No AES GCM AEAD supportTry installing gnutls if that does not support AEAD try installing nettle-dev or nettle-devel
samba-xc: [configure-native] failed './configure.developer --with-selftest-prefix=./bin/ab --picky-developer --prefix=/tmp/b9051/prefix/samba-xc --extra-python=/usr/bin/python3 --with-profiling-data' with status 1
~~~

This doesn't seem to have anything to do with the patch included here or in #115.