fix prototype for strsep

include/string.h

@@ -73,7 +73,7 @@ int strcasecmp (const char *, const char *);
 int strncasecmp (const char *, const char *, size_t);
 char *strchrnul(const char *, int);
 char *strcasestr(const char *, const char *);
-char *strsep(char **, char *);
+char *strsep(char **, const char *);
 #endif
 
 #ifdef __cplusplus

src/string/strsep.c

@@ -1,3 +1,4 @@
+#define _GNU_SOURCE
 #include <string.h>
 
 char *strsep(char **str, const char *sep)