Cleanup includes

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

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

app/src/v4l2_sink.c

@@ -1,5 +1,9 @@
 #include "v4l2_sink.h"
 
+#include <assert.h>
+#include <errno.h>
+#include <stdint.h>
+#include <stdlib.h>
 #include <string.h>
 
 #include "util/log.h"