workaround for non-DCFA make

suppress the following error:

ihk/ihk.o: In function `generic_common_handlers':
(.data+0x2678): undefined reference to `sys_mod_call'

arch/x86/kernel/syscall.c

@@ -20,6 +20,7 @@
 #include <syscall_list.h>
 #undef	SYSCALL_HANDLED
 #undef	SYSCALL_DELEGATED
+extern long sys_mod_call(int n, ihk_mc_user_context_t *ctx) __attribute__ ((weak));
 
 /* generate syscall_table[] */
 long (*syscall_table[])(int, ihk_mc_user_context_t *) = {