add: string.h

include/string.h

@@ -0,0 +1,9 @@
+#pragma once
+
+#include <stddef.h>
+
+char *strchr(const char *str, int c);
+int strcmp(const char *s1, const char *s2);
+int strncmp(const char *s1, const char *s2, size_t n);
+size_t strlen(const char *str);
+char *strstr(const char *haystack, const char *needle);