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>

include/asm-x86/e820.h

@@ -59,6 +59,7 @@ struct e820map {
 	struct e820entry map[E820_X_MAX];
 };
 
+#ifdef __KERNEL__
 /* see comment in arch/x86/kernel/e820.c */
 extern struct e820map e820;
 extern struct e820map e820_saved;
@@ -115,7 +116,7 @@ extern void setup_memory_map(void);
 extern char *default_machine_specific_memory_setup(void);
 extern char *machine_specific_memory_setup(void);
 extern char *memory_setup(void);
-
+#endif /* __KERNEL__ */
 #endif /* __ASSEMBLY__ */
 
 #define ISA_START_ADDRESS	0xa0000