docs: install libcrypt-devel on cygwin

this is included into Python.h, thus needed when compiling stuff.

docs/installation.rst

@@ -276,7 +276,7 @@ Cygwin
 
 Use the Cygwin installer to install the dependencies::
 
-    python3 python3-devel python3-setuptools
+    python3 python3-devel libcrypt-devel python3-setuptools
     binutils gcc-g++
     libopenssl openssl-devel
     git make openssh