speed up build by only including regex.h where needed

src/common.h

@@ -68,7 +68,6 @@
 #  include	<arpa/inet.h>
 #  include	<grp.h>
 #  include	<pwd.h>
-#  include      <regex.h>
 
 /* rest - some oddball headers */
 #ifdef HAVE_VALUES_H

src/conf.c

@@ -23,6 +23,7 @@
  * add new directives to.  Who knows if I'm right though.
  */
 
+#include <regex.h>
 #include "common.h"
 #include "conf.h"
 

src/filter.c

@@ -24,6 +24,7 @@
 
 #include "main.h"
 
+#include <regex.h>
 #include "filter.h"
 #include "heap.h"
 #include "log.h"

src/html-error.c

@@ -20,6 +20,7 @@
  * HTML error pages with variable substitution.
  */
 
+#include <regex.h>
 #include "main.h"
 
 #include "common.h"