env: Add support for callbacks to environment vars

Add support for per-variable callbacks to the "hashtable" functions.

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>

!!!fix comment in callback
diff --git a/include/environment.h b/include/environment.h
index 4b19f32..6c30215 100644
--- a/include/environment.h
+++ b/include/environment.h
@@ -164,6 +164,8 @@
 
 #ifndef DO_DEPS_ONLY
 
+#include <env_attr.h>
+#include <env_callback.h>
 #include <search.h>
 
 extern struct hsearch_data env_htab;