fifo.c: Fix -Wmissing-prototypes warnings.

src/libespeak-ng/fifo.c

 static void *say_thread(void *);
 
 static espeak_ng_STATUS push(t_espeak_command *the_command);
-static t_espeak_command *pop();
+static t_espeak_command *pop(void);
 static void init(int process_parameters);
 static int node_counter = 0;
 

src/libespeak-ng/fifo.h

 
 // Initialize the fifo component.
 // First function to be called.
-void fifo_init();
+void fifo_init(void);
 
 // Add an espeak command.
 //
 espeak_ng_STATUS fifo_add_commands(t_espeak_command *c1, t_espeak_command *c2);
 
 // The current running command must be stopped and the awaiting commands are cleared.
-espeak_ng_STATUS fifo_stop();
+espeak_ng_STATUS fifo_stop(void);
 
 // Is there a running command?
 // Returns 1 if yes; 0 otherwise.
-int fifo_is_busy();
+int fifo_is_busy(void);
 
 // Terminate the fifo component.
 // Last function to be called.
-void fifo_terminate();
+void fifo_terminate(void);
 
 // Indicates if the running command is still enabled.
 //
 // stopping speech as soon as a cancel command is applied.
 //
 // Returns 1 if yes, or 0 otherwise.
-int fifo_is_command_enabled();
+int fifo_is_command_enabled(void);
 
 #ifdef __cplusplus
 }