Add minor improvements and cleanup

libc/str/isascii.c

@@ -17,6 +17,7 @@
 │ Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA                │
 │ 02110-1301 USA                                                               │
 ╚─────────────────────────────────────────────────────────────────────────────*/
+#include "libc/str/str.h"
 
 int isascii(int c) {
   return 0x00 <= c && c <= 0x7F;