all: cleanup includes in header files

- don't include "nm-default.h" in header files. Every source file must
  include as first header "nm-default.h", thus our headers get the
  default include already implicitly.

- we don't support compiling NetworkManager itself with a C++ compiler. Remove
  G_BEGIN_DECLS/G_END_DECLS from internal headers. We do however support
  users of libnm to use C++, thus they stay in public headers.

(cherry picked from commit f19aff8909)

src/main-utils.h

@@ -21,8 +21,6 @@
 #ifndef __MAIN_UTILS_H__
 #define __MAIN_UTILS_H__
 
-#include "nm-default.h"
-
 void     nm_main_utils_ensure_root (void);
 
 void     nm_main_utils_setup_signals (GMainLoop *main_loop);