arm64: asm/perf_regs.h: Avoid C++-style comment in UAPI header

arm64: asm/perf_regs.h: Avoid C++-style comment in UAPI header

An arm64 'allmodconfig' build fails with GCC due to use of a C++-style
comment for the new SVE vector granule 'enum perf_event_arm_regs' entry:

  | /usr/include/asm/perf_regs.h:42:26: error: C++ style comments are not allowed in ISO C90

Use good ol' /* */ comment syntax to keep things rosey.

Link: https://lore.kernel.org/r/632cceb2.170a0220.599ec.0a3a@mx.google.com
Fixes: cbb0c02caf4b ("perf: arm64: Add SVE vector granule register to user regs")
Signed-off-by: Will Deacon <will@kernel.org>