include config header for use of CONFIG_PHAL_API

CONFIG_PHAL_API is defined in config.h, which was not included in
attn_handler.cpp before use of CONFIG_PHAL_API.

Change-Id: Ic5fb0695bc0811ffcd232511b52023e33469c43a
Signed-off-by: Zane Shelley <zshelle@us.ibm.com>
diff --git a/attn/attn_handler.cpp b/attn/attn_handler.cpp
index 718828a..67390d8 100644
--- a/attn/attn_handler.cpp
+++ b/attn/attn_handler.cpp
@@ -4,6 +4,8 @@
 #include <libpdbg_sbe.h>
 }
 
+#include <config.h>
+
 #ifdef CONFIG_PHAL_API
 #include <libphal.H>
 #endif