perf: add arm64 source files for unistd_64.h

perf: add arm64 source files for unistd_64.h

kernel commit bfb713ea53c7 ("perf tools: Fix arm64 build by generating unistd_64.h")
introduces a new dependency on source files for arm64, specifically
include/uapi/asm-generic.

Build fails with:
[..]/perf/1.0/perf-1.0/scripts/Makefile.asm-headers:33: [...]/perf/1.0/perf-1.0/include/uapi/asm-generic/Kbuild: No such file or directory
make[4]: *** No rule to make target '[...]/perf/1.0/perf-1.0/include/uapi/asm-generic/Kbuild'.  Stop.

Add the directory to PERF_SRC.
Fix whitespace error while at it.

Signed-off-by: Yannic Moog <y.moog@phytec.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>