hush: plug the leak of expanded heredoc

diff --git a/shell/math.c b/shell/math.c
index 8a44c8e..cc298bd 100644
--- a/shell/math.c
+++ b/shell/math.c
@@ -26,8 +26,7 @@
  * rewrote arith (see notes to this), added locale support,
  * rewrote dynamic variables.
  */
-
-#include "busybox.h"
+#include "libbb.h"
 #include "math.h"
 
 #define a_e_h_t arith_eval_hooks_t