html-error: move common.h inclusion back to top

this seems to cause an implicit declaration of snprintf() thanks to
feature test macro hell.

src/html-error.c

@@ -20,10 +20,9 @@
  * HTML error pages with variable substitution.
  */
 
-#include <regex.h>
+#include "common.h"
 #include "main.h"
 
-#include "common.h"
 #include "buffer.h"
 #include "conns.h"
 #include "heap.h"
@@ -32,6 +31,8 @@
 #include "utils.h"
 #include "conf.h"
 
+#include <regex.h>
+
 /*
  * Add an error number -> filename mapping to the errorpages list.
  */