Use (void) for function prototypes with no parameters

This is required when compiling these headers as C code. These were
omitted becuase before the addition of examples these headers were
only compiled locally as C++ files where adding the void keyword is
not needed.

include/SFML/Window/Clipboard.h

@@ -42,7 +42,7 @@
 /// \return Clipboard contents as a locale-dependent ANSI string
 ///
 ////////////////////////////////////////////////////////////
-CSFML_WINDOW_API const char* sfClipboard_getString();
+CSFML_WINDOW_API const char* sfClipboard_getString(void);
 
 
 ////////////////////////////////////////////////////////////
@@ -55,7 +55,7 @@ CSFML_WINDOW_API const char* sfClipboard_getString();
 /// \return Clipboard contents as UTF-32
 ///
 ////////////////////////////////////////////////////////////
-CSFML_WINDOW_API const sfUint32* sfClipboard_getUnicodeString();
+CSFML_WINDOW_API const sfUint32* sfClipboard_getUnicodeString(void);
 
 ////////////////////////////////////////////////////////////
 /// \brief Set the content of the clipboard as ANSI string data

include/SFML/Window/Context.h

@@ -32,7 +32,7 @@
 #include <SFML/Window/Types.h>
 #include <SFML/Window/Window.h>
 
-typedef void (*GlFunctionPointer)();
+typedef void (*GlFunctionPointer)(void);
 
 ////////////////////////////////////////////////////////////
 /// \brief Create a new context
@@ -104,6 +104,6 @@ CSFML_WINDOW_API sfContextSettings sfContext_getSettings(const sfContext* contex
 /// \return The active context's ID or 0 if no context is currently active
 ///
 ////////////////////////////////////////////////////////////
-CSFML_WINDOW_API sfUint64 sfContext_getActiveContextId();
+CSFML_WINDOW_API sfUint64 sfContext_getActiveContextId(void);
 
 #endif // SFML_CONTEXT_H

include/SFML/Window/Vulkan.h

@@ -43,7 +43,7 @@ typedef uint64_t VkSurfaceKHR;
 typedef struct VkAllocationCallbacks VkAllocationCallbacks;
 
 
-typedef void (*sfVulkanFunctionPointer)();
+typedef void (*sfVulkanFunctionPointer)(void);
 
 ////////////////////////////////////////////////////////////
 /// \brief Tell whether or not the system supports Vulkan
@@ -79,4 +79,4 @@ CSFML_WINDOW_API sfVulkanFunctionPointer sfVulkan_getFunction(const char* name);
 /// \return Vulkan instance extensions required for graphics
 ///
 ////////////////////////////////////////////////////////////
-CSFML_WINDOW_API const char* const* sfVulkan_getGraphicsRequiredInstanceExtensions();
+CSFML_WINDOW_API const char* const* sfVulkan_getGraphicsRequiredInstanceExtensions(void);