x86/alternatives: add missing insn.h include

From: Dave Hansen <dave.hansen@linux.intel.com>

While testing my MPX removal series, Borislav noted compilation
failure with an allnoconfig build.

Turned out to be a missing include of insn.h in alternative.c.
With MPX, it got it implicitly from:

	asm/mmu_context.h -> asm/mpx.h -> asm/insn.h

Fixes: c3d6324f ("x86/alternatives: Teach text_poke_bp() to emulate instructions")
Reported-by: Borislav Petkov <bp@alien8.de>
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: x86@kernel.org
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>