Force the source_dir of ctpp2 source.

The `source_dir` is automatically infer from the name of the dependency.
As the ctpp2 source is use in two dependencies (ctpp2 and ctpp2c), if
we use the default source_dir, kiwix-build will use one name and the
other dependency will not find the source directory.

By forcing the `source_dir`, both dependencies will use the same directory.

dependencies.py

@@ -135,6 +135,8 @@ class CTPP2(Dependency):
     version = "2.8.3"
 
     class Source(ReleaseDownload):
+        name = "ctpp2"
+        source_dir = "ctpp2-2.8.3"
         archive = Remotefile('ctpp2-2.8.3.tar.gz',
                              'a83ffd07817adb575295ef40fbf759892512e5a63059c520f9062d9ab8fb42fc')
         patches = ["ctpp2_include.patch",