Avoid the missing-prototypes warning

Warning option -Wmissing-prototypes raises a warning for all functions
that are neither static nor in a header. Some functions should be available
only for main.c but not in the official header (civetweb.h) since they are
subject to changes. In order to avoid this warning, additional headers
outside the include folder must be added.

src/main.c

@@ -641,7 +641,8 @@ static void set_absolute_path(char *options[],
 #ifdef USE_LUA
 #include "lua.h"
 #include "lauxlib.h"
-extern void lua_civet_open_all_libs(lua_State *L);
+#include "lua_civet.h"
+
 
 static int run_lua(const char *file_name)
 {