PR32930 backends/: guard asm/perf_regs.h include

PR32930 backends/: guard asm/perf_regs.h include

asm/perf_regs.h is an arch-specific linux include, not present on
architectures like hppa and m68k that lack perf_events support.

Only one place we need to fix; others already guard the include by
architecture, or use architecture-independent headers (e.g.
linux/perf_events.h).

* backends/libebl_PERF_FLAGS.h: Only include asm/perf_regs.h on
  architectures where we use it.

Signed-off-by: Serhei Makarov <serhei@serhei.io>