xen64: fix build error on 32-bit + !HIGHMEM

fix:

arch/x86/xen/enlighten.c: In function 'xen_set_fixmap':
arch/x86/xen/enlighten.c:1127: error: 'FIX_KMAP_BEGIN' undeclared (first use in this function)
arch/x86/xen/enlighten.c:1127: error: (Each undeclared identifier is reported only once
arch/x86/xen/enlighten.c:1127: error: for each function it appears in.)
arch/x86/xen/enlighten.c:1127: error: 'FIX_KMAP_END' undeclared (first use in this function)
make[1]: *** [arch/x86/xen/enlighten.o] Error 1
make: *** [arch/x86/xen/enlighten.o] Error 2

FIX_KMAP_BEGIN is only available on HIGHMEM.
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/xen/enlighten.c

@@ -1124,7 +1124,9 @@ static void xen_set_fixmap(unsigned idx, unsigned long phys, pgprot_t prot)
 #ifdef CONFIG_X86_32
 	case FIX_WP_TEST:
 	case FIX_VDSO:
+# ifdef CONFIG_HIGHMEM
 	case FIX_KMAP_BEGIN ... FIX_KMAP_END:
+# endif
 #else
 	case VSYSCALL_LAST_PAGE ... VSYSCALL_FIRST_PAGE:
 #endif