ftw: Fix incompatible pointer types error

This fixes a compile error with GCC 15.

Author: sebhub

newlib/libc/posix/ftw.c

@@ -30,7 +30,9 @@ int ftw(const char *path, int (*fn)(const char *, const struct stat *, int), int
 	/* The following cast assumes that calling a function with one
 	 * argument more than it needs behaves as expected. This is
 	 * actually undefined, but works on all real-world machines. */
-	return nftw(path, (int (*)())fn, fd_limit, FTW_PHYS);
+	return nftw(path,
+	    (int (*)(const char *, const struct stat *, int,  struct FTW *))fn,
+	    fd_limit, FTW_PHYS);
 }
 
 #endif /* ! HAVE_OPENDIR */