um: don't generate asm/bpf_perf_event.h

If we start validating the existence of the asm-generic side of
generated headers, this one causes a warning:

make[3]: *** No rule to make target 'arch/um/include/generated/asm/bpf_perf_event.h', needed by 'all'.  Stop.

The problem is that the asm-generic header only exists for the uapi
variant, but arch/um has no uapi headers and instead uses the x86
userspace API.

Add a custom file with an explicit redirect to avoid this.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>