grit helped me

Author: Jolah1

0x09-static_libraries/main.h

@@ -1,3 +1,6 @@
+#ifndef MAIN_H
+#define MAIN_H
+
 int _putchar(char c);
 int _islower(int c);
 int _isalpha(int c);
@@ -18,3 +21,5 @@ char *_strchr(char *s, char c);
 unsigned int _strspn(char *s, char *accept);
 char *_strpbrk(char *s, char *accept);
 char *_strstr(char *haystack, char *needle);
+
+#endif