[PATCH] Always #include corresponding .h in .c files

[Jason A. Donenfeld]

Making this change also requires including stddef.h in html.h, lest
there's a compiler error because size_t isn't defined. I'll fold this
patch into yours and put it in wip.

diff --git a/html.h b/html.h
index b89cf94..bb36f37 100644
--- a/html.h
+++ b/html.h
@@ -1,6 +1,8 @@
 #ifndef HTML_H
 #define HTML_H

+#include <stddef.h>
+
 extern void html_raw(const char *txt, size_t size);
 extern void html(const char *txt);