Add mman.h for Windows compatibility

[N-V-R]

When I do 'gem install scrypt' on Windows 7, there is an error:

ERROR: Error installing scrypt:
ERROR: Failed to build gem native extension.

current directory: C:/Ruby/lib/ruby/gems/2.3.0/gems/scrypt-3.0.0/ext/scrypt
C:/Ruby/bin/ruby.exe -rubygems C:/Ruby/lib/ruby/gems/2.3.0/gems/rake-11.2.2/exe/rake RUBYARCHDIR=C:/Ruby/lib/ruby/gems/2.3.0/extensions/x86-mingw32/2.3.0/scrypt-3.0.0 RUBYLIBDIR=C:/Ruby/lib/ruby/gems/2.3.0/extensions/x86-mingw32/2.3.0/scrypt-3.0.0
mkdir -p i386-windows
gcc -fexceptions -O -fno-omit-frame-pointer -fno-strict-aliasing -Wall -std=c99 -msse -msse2 -D_GNU_SOURCE=1 -fPIC  -o i386-windows/crypto_scrypt.o -c ./crypto_scrypt.c
./crypto_scrypt.c:1:0: warning: -fPIC ignored for target (all code is position independent) [enabled by default]
./crypto_scrypt.c:32:22: fatal error: sys/mman.h: No such file or directory compilation terminated.
rake aborted!
Command failed with status (1): [gcc -fexceptions -O -fno-omit-frame-pointe...]

Tasks: TOP => default => i386-windows/scrypt_ext.dll => i386-windows/crypto_scrypt.o
(See full trace by running task with --trace)

rake failed, exit code 1

Gem files will remain installed in C:/Ruby/lib/ruby/gems/2.3.0/gems/scrypt-3.0.0 for inspection.
Results logged to C:/Ruby/lib/ruby/gems/2.3.0/extensions/x86-mingw32/2.3.0/scrypt-3.0.0/gem_make.out

So, I added a mman.h to fix (source: https://github.com/witwall/mman-win32).

Note: actually there is another error on Windows, with ffi gem (LoadError: cannot load such file -- ffi_c). You can fix it by gem uninstall ffi -I && gem install ffi --platform=ruby.

[stakach]

I haven't cut the GEM yet as whilst I am now a core member of the FFI team I'm waiting on the original owner of ffi-compiler to add me to the rubygems project.
The release should be out in a couple of days at most.

[stakach]

@N-V-R OK the new release 3.0.1 is cut. Could you please test?

[N-V-R]

Now everything is fine (with preinstalled ffi: gem install ffi --platform=ruby). Thank you!