x86/gsseg: Add the new <asm/gsseg.h> header to <asm/asm-prototypes.h>

Module build needs to be able to pick up the C prototype:

  WARNING: modpost: EXPORT symbol "asm_load_gs_index" [vmlinux] version generation failed, symbol will not be versioned.
  Is "asm_load_gs_index" prototyped in <asm/asm-prototypes.h>?

Fixes: ae53fa18

 ("x86/gsseg: Move load_gs_index() to its own new header file")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: linux-kernel@vger.kernel.org