It fixes for compiling error on C++20.

[mura-orz]

If it changes the version of CMAKE_CXX_STANDARD to 20, the following errors occurred by g++11.4.0.

In file included from ~/cpp-peglib/test/test1.cc:2:
~/cpp-peglib/test/../peglib.h: In static member function ‘static bool peg::ParserGenerator::parse_test(const char8_t*, const char8_t*)’:
~/cpp-peglib/test/../peglib.h:3324:5: error: ‘reutnr’ was not declared in this scope
 3324 |     reutnr parse_test(reinterpret_cast<const char *>(s), s);
      |     ^~~~~~
~/cpp-peglib/test/../peglib.h:3325:3: warning: no return statement in function returning non-void [-Wreturn-type]
 3325 |   }
      |   ^
~/cpp-peglib/test/../peglib.h:3323:41: warning: unused parameter ‘d’ [-Wunused-parameter]
 3323 |   static bool parse_test(const char8_t *d, const char8_t *s) {
      |                          ~~~~~~~~~~~~~~~^
~/cpp-peglib/test/../peglib.h:3323:59: warning: unused parameter ‘s’ [-Wunused-parameter]
 3323 |   static bool parse_test(const char8_t *d, const char8_t *s) {
      |                                            ~~~~~~~~~~~~~~~^
make[2]: *** [test/CMakeFiles/peglib-test-main.dir/build.make:76: test/CMakeFiles/peglib-test-main.dir/test1.cc.o] Error 1
make[1]: *** [CMakeFiles/Makefile2:152: test/CMakeFiles/peglib-test-main.dir/all] Error 2

[yhirose]

@mura-orz thanks for catching my mistake!