build: Copy extra sources to build directory during %prep

The Debian package builder makes a tarball from the build directory
after executing the %prep stage, so we must copy all extra sources
into the build directory during this stage.

Signed-off-by: Euan Harris <euan.harris@citrix.com>
Merge pull request #148 from euanh/build-improvements

build: Copy extra sources to build directory during %prep