Build fix

I got the following build error. The problem seems to be that both `linux/perf_event.h` and `perfmon/perf_event.h` are included. The solution is not to include `linux/perf_event.h`.

This is with libpfm-4.4.0 (Ubuntu 13.10).

```
In file included from /usr/include/perfmon/pfmlib_perf_event.h:26:0,
                 from /home/tinuviel/github/mozilla/rr/src/hpc.cc:8:
/usr/include/perfmon/perf_event.h:431:7: error: redefinition of ‘union perf_mem_data_src’
 union perf_mem_data_src {
       ^
In file included from /home/tinuviel/github/mozilla/rr/src/hpc.h:12:0,
                 from /home/tinuviel/github/mozilla/rr/src/hpc.cc:3:
/usr/include/linux/perf_event.h:627:7: error: previous definition of ‘union perf_mem_data_src’
 union perf_mem_data_src {
       ^
```