[librm] Add missing __asmcall on init_idt()

The __asmcall declaration has no effect on a void function with no
parameters, but should be included for completeness since the function
is called directly from assembly code.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/x86/transitions/librm_mgmt.c

@@ -118,7 +118,7 @@ void set_interrupt_vector ( unsigned int intr, void *vector ) {
  * Initialise interrupt descriptor table
  *
  */
-void init_idt ( void ) {
+__asmcall void init_idt ( void ) {
 	struct interrupt_vector *vec;
 	unsigned int intr;