Cleanup includes

Improved manually with the help of neovim LSP warnings and iwyu:

    iwyu -Ibuilddir/app/ -Iapp/src/ app/src/XXX.c

app/src/util/strbuf.c

@@ -1,11 +1,10 @@
 #include "strbuf.h"
 
 #include <assert.h>
-#include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 
-#include "log.h"
+#include "util/log.h"
 
 bool
 sc_strbuf_init(struct sc_strbuf *buf, size_t init_cap) {