gen_syscall_header.py: fix include issue

This header uses definitions in include/syscall.h but wasn't
explicitly including it.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>
diff --git a/scripts/gen_syscall_header.py b/scripts/gen_syscall_header.py
index 6eab116..f4a66dd 100755
--- a/scripts/gen_syscall_header.py
+++ b/scripts/gen_syscall_header.py
@@ -149,6 +149,7 @@
 sys.stdout.write(
     "/* Auto-generated by gen_syscall_header.py, do not edit! */\n\n")
 sys.stdout.write("#ifndef GEN_SYSCALL_H\n#define GEN_SYSCALL_H\n\n")
+sys.stdout.write("#include <syscall.h>\n")
 
 for i in range(11):
     sys.stdout.write(