LibCore: Rename puff.c => puff.cpp

Now that we don't keep a C compiler around in the toolchain (to save
space) we can't have .c files in the build.

This reminds me that #362 exists and we should fix that at some point.

Libraries/LibCore/CMakeLists.txt

@@ -18,7 +18,7 @@ set(SOURCES
     Notifier.cpp
     Object.cpp
     ProcessStatisticsReader.cpp
-    puff.c
+    puff.cpp
     SocketAddress.cpp
     Socket.cpp
     StandardPaths.cpp