freadln.c: add missing inclusion of string.h

c-programming/io/freadln.c

@@ -86,6 +86,7 @@ int freadln(FILE* f, char** output, size_t* length_out) {
 # include <inttypes.h>
 # include <stdbool.h>
 # include <stdint.h>
+# include <string.h>
 
 # if POSIX
 #  include <unistd.h>