Fix zlib source directory name.

This fixes the non-bootstrap ARM build.
I'm not sure this is the right fix, the zlib package seems to come with a
"source" (not "sources") folder on both ARM and x86_gcc2 but then I
don't understand how this worked for the x86_gcc2 build before.

build/jam/BuildFeatures

@@ -438,7 +438,7 @@ if [ IsPackageAvailable zlib_devel ] {
 		# sources are required for the primary architecture only
 		primary @{
 			file: source zlib_source
-				sources: develop/sources/%portRevisionedName%/sources
+				sources: develop/sources/%portRevisionedName%/source
 		}@
 		;