Fix strict prototype clang warning

cmark-gfm/src/node.h:122:36: warning: this function declaration is not a prototype [-Wstrict-prototypes]
void cmark_init_standard_node_flags();
                                   ^
                                    void
1 warning generated.

Author: huven

src/node.h

@@ -119,7 +119,7 @@ void cmark_register_node_flag(cmark_node_internal_flags *flags);
  * library. It is now a no-op.
  */
 CMARK_GFM_EXPORT
-void cmark_init_standard_node_flags();
+void cmark_init_standard_node_flags(void);
 
 static CMARK_INLINE cmark_mem *cmark_node_mem(cmark_node *node) {
   return node->content.mem;