um/x86: Add system call table to header file

The generic system call tracing infrastructure requires access to the
system call table. The symbol is already visible to the linker but is
lacking a public declaration.

Add a public declaration.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Reviewed-by: Nam Cao <namcao@linutronix.de>
Link: https://patch.msgid.link/20250703-uml-have_syscall_tracepoints-v1-1-23c1d3808578@linutronix.de


Signed-off-by: Johannes Berg <johannes.berg@intel.com>