wildtest: use stdbool.h so bool works from C99 through C23

The Cygwin CI runner's autoconf selects gcc -std=gnu23, where bool is
a keyword, so 'typedef char bool' fails with 'two or more data types
in declaration specifiers'.  Guarding the typedef on __STDC_VERSION__
is unreliable across the C2x transition (that compiler reports a
pre-release value below 202311L), so drop the typedef and include
stdbool.h, which works from C99 through C23.

wildtest.c

@@ -32,9 +32,7 @@ int fnmatch_errors = 0;
 
 int wildmatch_errors = 0;
 
-#if !defined(__STDC_VERSION__) || __STDC_VERSION__ < 202311L
-typedef char bool;
-#endif
+#include <stdbool.h>
 
 int output_iterations = 0;
 int explode_mod = 0;