MXS-2197: Make config.h and service.h C++ headers

This is the first step into converting the other headers into C++.

include/maxscale/filter.h

@@ -20,13 +20,11 @@
 #include <stdint.h>
 #include <maxbase/jansson.h>
 #include <maxscale/buffer.h>
-#include <maxscale/config.h>
+#include <maxscale/config.hh>
 #include <maxscale/dcb.h>
 #include <maxscale/routing.h>
 #include <maxscale/session.h>
 
-MXS_BEGIN_DECLS
-
 /**
  * MXS_FILTER is an opaque type representing a particular filter instance.
  *
@@ -264,5 +262,3 @@ typedef enum filter_capability
 {
     FCAP_TYPE_NONE = 0x0    // TODO: remove once filter capabilities are defined
 } filter_capability_t;
-
-MXS_END_DECLS