demo: Avoid debug message in demo/src/lib/mini_c

The "strncpy - not implemented" message appeared on the use of libpng.
Even though it did not cause trouble, spilling the log with such
easy-to-fix issues should better be avoided.

demo/src/lib/mini_c/mini_c.c

@@ -66,7 +66,7 @@ void *calloc(unsigned nmemb, unsigned size)
 void *memcpy(void *dest, const void *src, unsigned n);
 */
 char *strcpy(char *dest, const char *src)
-	{ printf("%s: not implemented\n", __func__); return 0; }
+	{ return strncpy(dest, src, ~0); }
 char *strcat(char *dest, const char *src)
 	{ printf("%s: not implemented\n", __func__); return 0; }