[libngf] Do not use C++ style function prototypes. Fixes JB#32195

Incorrect function prototypes in libngfd causes compilation warnings
when strict error checking is used.

Declare functions that take no parameters as
  int foo(void);
instead of
  int foo();

The latter is valid in C++, but in C compilation it is "old style"
prototype meaning the number/type of parameters is not specified.

libngf/proplist.h

@@ -49,7 +49,7 @@ typedef void    (*NgfProplistExtendedCallback) (const char *key, const void *val
  * @return NgfProplist or NULL if no memory.
  */
 
-NgfProplist*    ngf_proplist_new ();
+NgfProplist*    ngf_proplist_new (void);
 
 /**
  * Create an identical copy of other proplist.