Add '#!/bin/sh' to cygwin-package.sh

This commit adds '#!/bin/sh' to the top of the cygwin-package.sh.in
script.  All executable interpreted scripts should have this directive.

Patch from Modestas Vainius.  See issue #9659.

Templates/cygwin-package.sh.in

@@ -1,3 +1,5 @@
+#!/bin/sh
+
 # this is a sample shell script used for building a cmake
 # based project for a cygwin setup package.