Bug 1556662 - Fix a typo in change from bug 1469091. r=dmajor

In a branch for `isinstance(obj, (HostSources, HostGeneratedSources))`,
`isinstance(obj, GeneratedSources)` is clearly not going to match
anything. `isinstance(obj, HostGeneratedSources)` is what was intended.

Differential Revision: https://phabricator.services.mozilla.com/D33603

--HG--
extra : moz-landing-system : lando

python/mozbuild/mozbuild/backend/recursivemake.py

@@ -485,7 +485,7 @@ class RecursiveMakeBackend(CommonBackend):
                 '.cpp': 'HOST_CPPSRCS',
             }
             variables = [suffix_map[obj.canonical_suffix]]
-            if isinstance(obj, GeneratedSources):
+            if isinstance(obj, HostGeneratedSources):
                 variables.append('GARBAGE')
                 base = backend_file.objdir
             else: