Provide a global counter for the number of interrupts hooked.

src/arch/i386/include/biosint.h

@@ -8,6 +8,7 @@
 
 struct segoff;
 
+extern int hooked_bios_interrupts;
 extern void hook_bios_interrupt ( unsigned int interrupt, unsigned int handler,
 				  struct segoff *chain_vector );
 extern int unhook_bios_interrupt ( unsigned int interrupt,