capi: add missing void

Apparently in C, an empty parameter list means "the function takes an
unspecified number of arguments." (lol.) But an explicit void means
"the function takes zero arguments." The latter is indeed what we want
here.

Ref: https://softwareengineering.stackexchange.com/questions/286490/what-is-the-difference-between-function-and-functionvoid

Closes #942

regex-capi/include/rure.h

@@ -408,7 +408,7 @@ size_t rure_captures_len(rure_captures *captures);
  * safe to call rure_compile from multiple threads simultaneously using the
  * same options pointer.
  */
-rure_options *rure_options_new();
+rure_options *rure_options_new(void);
 
 /*
  * rure_options_free frees the given options.
@@ -536,7 +536,7 @@ size_t rure_set_len(rure_set *re);
  * It is not safe to use errors from multiple threads simultaneously. An error
  * value may be reused on subsequent calls to rure_compile.
  */
-rure_error *rure_error_new();
+rure_error *rure_error_new(void);
 
 /*
  * rure_error_free frees the error given.