Only include color.h in source files that require it

Remove the inclusion of color.h from config.h, which is included by every file via irc.h, and instead
include it only in debug.c, fset.c and vars.c which are the only files that use it.

This minimises the number of files that need to be rebuilt when changing default format strings.

source/debug.c

@@ -24,6 +24,7 @@ CVS_REVISION(debug_c)
 #include "vars.h"
 #define MAIN_SOURCE
 #include "modval.h"
+#include "color.h"
 
 unsigned long x_debug = 0;
 unsigned long internal_debug = 0;