fix compiler warning
fix compiler warning

The patch fixed the following compiler warnings:

  /home/yhs/work/bcc/src/cc/perf_reader.c: In function ‘read_data_head’:
  /home/yhs/work/bcc/src/cc/perf_reader.c:149:3: warning: dereferencing type-punned pointer will break strict-alias
  ing rules [-Wstrict-aliasing]
     uint64_t data_head = *((volatile uint64_t *)&perf_header->data_head);
     ^
  /home/yhs/work/bcc/src/cc/perf_reader.c: In function ‘read_data_head’:
  /home/yhs/work/bcc/src/cc/perf_reader.c:149:3: warning: dereferencing type-punned pointer will break strict-alias
  ing rules [-Wstrict-aliasing]
     uint64_t data_head = *((volatile uint64_t *)&perf_header->data_head);
     ^

Declaring perf_header as volatile type will force its member read from memory,
hence avoiding a forced type conversion.

Signed-off-by: Yonghong Song <e76892bbe783db2f627de1232e139db4de43da60@fb.com>

tuxology/bcc,tuxology/bcc,mcaleavya/bcc,mcaleavya/bcc,iovisor/bcc,brendangregg/bcc,mcaleavya/bcc,tuxology/bcc,tuxology/bcc,brendangregg/bcc,brendangregg/bcc,mcaleavya/bcc,iovisor/bcc,iovisor/bcc,brendangregg/bcc,tuxology/bcc,iovisor/bcc,iovisor/bcc,brendangregg/bcc,mcaleavya/bcc