gen_syscalls.py: fix include issue

The generated header uses Zephyr's custom integer type
definitions but wasn't including the proper header.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>
diff --git a/scripts/gen_syscalls.py b/scripts/gen_syscalls.py
index 750d82d..ee9fadd 100755
--- a/scripts/gen_syscalls.py
+++ b/scripts/gen_syscalls.py
@@ -29,6 +29,8 @@
 
 #ifndef _ASMLANGUAGE
 
+#include <zephyr/types.h>
+
 #ifdef __cplusplus
 extern "C" {
 #endif