x86/dumpstack: Unexport oops_begin()

The only user outside of arch/ is not a module since

  86cd4733 ("ACPI, APEI, GHES, Prevent GHES to be built as module")

No functional changes.
Signed-off-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Link: https://lkml.kernel.org/r/20180417161124.5294-3-bp@alien8.de

arch/x86/kernel/dumpstack.c

@@ -268,7 +268,6 @@ unsigned long oops_begin(void)
 	bust_spinlocks(1);
 	return flags;
 }
-EXPORT_SYMBOL_GPL(oops_begin);
 NOKPROBE_SYMBOL(oops_begin);
 
 void __noreturn rewind_stack_do_exit(int signr);