x86: move extern declaration to vdso.h

Before:
   total: 0 errors, 3 warnings, 685 lines checked
After:
   total: 0 errors, 1 warnings, 678 lines checked

No code changed:

arch/x86/kernel/signal_32.o:

   text	   data	    bss	    dec	    hex	filename
   5333	      0	      4	   5337	   14d9	signal_32.o.before
   5333	      0	      4	   5337	   14d9	signal_32.o.after

md5:
   c279e98012a2808e90cfa2a7787e42a4  signal_32.o.before.asm
   c279e98012a2808e90cfa2a7787e42a4  signal_32.o.after.asm

Signed-off-by: Ingo Molnar <mingo@elte.hu>