hw-breakpoints: Fix build on !perf architectures

the arch/alpha build fails with:

 In file included from tip/kernel/exit.c:52:
 tip/include/linux/hw_breakpoint.h: In function 'hw_breakpoint_addr':
 tip/include/linux/hw_breakpoint.h:21: error: 'struct perf_event' has no member named 'attr'
 [...]

Move these helper inlines inside the CONFIG_HAVE_HW_BREAKPOINT ifdef.

Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Prasad <prasad@linux.vnet.ibm.com>
LKML-Reference: <1258114575-32655-1-git-send-email-fweisbec@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>