build: assume errno.h is always available

- Remove errno.h detection from all build configurations.

errno.h is a standard header according to C89.

Closes https://github.com/curl/curl/pull/9986

lib/curl_setup_once.h

@@ -34,10 +34,7 @@
 #include <string.h>
 #include <stdarg.h>
 #include <time.h>
-
-#ifdef HAVE_ERRNO_H
 #include <errno.h>
-#endif
 
 #ifdef HAVE_SYS_TYPES_H
 #include <sys/types.h>