x86, bts: fix build error

Impact: build fix

 arch/x86/kernel/ds.c: In function 'ds_request':
 arch/x86/kernel/ds.c:236: sorry, unimplemented: inlining failed in call to 'ds_get_context': recursive inlining

but the recursion here is scary ...

Signed-off-by: Ingo Molnar <mingo@elte.hu>