pkg/wuffs: build without libc

This modifies our wuffs dependency so that it no longer requires libc.

This unblocks using wuffs from libghostty-vt on freestanding targets,
which we'll eventually want for some Kitty graphics stuff.

pkg/wuffs/include/stdlib.h

@@ -0,0 +1,13 @@
+// Minimal stdlib.h so that wuffs can be translated and compiled without
+// requiring libc headers. See string.h in this directory for details.
+#ifndef GHOSTTY_WUFFS_STDLIB_H
+#define GHOSTTY_WUFFS_STDLIB_H
+
+#include <stddef.h>
+
+void *malloc(size_t size);
+void *calloc(size_t count, size_t size);
+void *realloc(void *ptr, size_t size);
+void free(void *ptr);
+
+#endif