IA32 emulation: Fix build problem for modular ia32 a.out support

Commit ce7e5d2d

 ("x86: fix broken TASK_SIZE for ia32_aout") breaks
kernel builds when "CONFIG_IA32_AOUT=m" with

  ERROR: "set_personality_ia32" [arch/x86/ia32/ia32_aout.ko] undefined!
  make[1]: *** [__modpost] Error 1

The entry point needs to be exported.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Acked-by: Al Viro <viro@zeniv.linux.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>