syscalls: skip generating mrsh.c if not userspace

There is no need to generate all the *_mrsh.c files for
marshalling syscall arguments when userspace is not enabled.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>

CMakeLists.txt

@@ -735,6 +735,7 @@ add_custom_command(OUTPUT include/generated/syscall_dispatch.c ${syscall_list_h}
   --base-output      include/generated/syscalls           # Write to this dir
   --syscall-dispatch include/generated/syscall_dispatch.c # Write this file
   --syscall-list     ${syscall_list_h}
+  $<$<BOOL:${CONFIG_USERSPACE}>:--gen-mrsh-files>
   ${SYSCALL_LONG_REGISTERS_ARG}
   ${SYSCALL_SPLIT_TIMEOUT_ARG}
   WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}