clang: Fix file_exists_and_ownedby return value

[luochenglcs]

commit 008ea09 (clang: check header ownership) updates file_exists() to file_exists_and_ownedby(), add verifies onwer, but the return value is different from before, causing problems with the original code.

After commit 008ea09 (clang: check header ownership)， /usr/share/bcc/examples/hello_world.py --help output is different: (left: old | right: new)

Because commit 008ea09 changed the return value：
Old: If file exists, file_exists() return 1;

static inline int file_exists(const char *f)
{
  struct stat buffer;
  return (stat(f, &buffer) == 0);  //ret = stat(f, &buffer) == 0
}

New: If file exists, file_exists_and_ownedby() return 0;

static inline int file_exists_and_ownedby(const char *f, uid_t uid)
{
  struct stat buffer;
  int ret;
  if ((ret = stat(f, &buffer)) == 0) {   //ret = stat(f, &buffer)) 
    if (buffer.st_uid != uid) {
      std::cout << "ERROR: header file ownership unexpected: " << std::string(f) << "\n";
      return -1;
    }
  }
  return ret;
}

[luochenglcs]

@brendangregg Please check.