scripts: gen_syscalls: typo fix

Fix "defing" as "defining"

Signed-off-by: Ruslan Mstoi <ruslan.mstoi@intel.com>
diff --git a/scripts/gen_syscalls.py b/scripts/gen_syscalls.py
index 3253155..6d18cf9 100755
--- a/scripts/gen_syscalls.py
+++ b/scripts/gen_syscalls.py
@@ -14,7 +14,7 @@
   as well as the system call dispatch table, which maps system call type IDs
   to their handler functions.
 
-- A header file defing the system call type IDs, as well as function
+- A header file defining the system call type IDs, as well as function
   prototypes for all system call handler functions.
 
 - A directory containing header files. Each header corresponds to a header