glib-genmarshal: wrap prototypes in G_{BEGIN,END}_DECLS.

Since --header --body has been deprecated and replaced with --body
--prototypes, the generated body is not wrapped with G_{BEGIN,END}_DECLS
any longer. Projects using C++ break due to that. Automatically wrap
prototypes individually in G_{BEGIN,END}_DECLS instead.

https://bugzilla.gnome.org/show_bug.cgi?id=785554

gobject/glib-genmarshal.in

@@ -556,7 +556,7 @@ def generate_prototype(retval, params,
                        va_marshal=False):
     '''Generate a marshaller declaration with the given @visibility. If @va_marshal
     is True, the marshaller will use variadic arguments in place of a GValue array.'''
-    signature = []
+    signature = ['G_BEGIN_DECLS']
 
     if visibility == Visibility.INTERNAL:
         signature += ['G_GNUC_INTERNAL']
@@ -585,6 +585,8 @@ def generate_prototype(retval, params,
         signature += [indent('int       n_params,', level=width, fill=' ')]
         signature += [indent('GType    *param_types);', level=width, fill=' ')]
 
+    signature += ['G_END_DECLS']
+
     return signature