Fix this GCC 4.6 warning with 64-bit builds.
u_debug_stack.c: In function ‘debug_backtrace_capture’:
u_debug_stack.c:45:17: warning: variable ‘frame_pointer’ set but not
used [-Wunused-but-set-variable]
Signed-off-by: Vinson Lee <vlee@freedesktop.org> Reviewed-by: José Fonseca <jfonseca@vmware.com>