x86: fix asm/e820.h for userspace inclusion

asm-x86/e820.h is included from userspace.  'x86: make e820.c to have
common functions' (b79cd8f1) broke it:

	make -C Documentation/lguest
	cc -Wall -Wmissing-declarations -Wmissing-prototypes -O3 -I../../include
lguest.c  -lz -o lguest
	In file included from ../../include/asm-x86/bootparam.h:8,
	                 from lguest.c:45:
	../../include/asm/e820.h:66: error: expected ‘)’ before ‘start’
	../../include/asm/e820.h:67: error: expected ‘)’ before ‘start’
	../../include/asm/e820.h:68: error: expected ‘)’ before ‘start’
	../../include/asm/e820.h:72: error: expected ‘=’, ‘,’, ‘;’, ‘asm’
or ‘__attribute__’ before ‘e820_update_range’
	...

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Cc: Yinghai Lu <yhlu.kernel@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>