Close directory after use in read_all_proc_fds.

Visible in `rr record detach_state` with asan enabled rr build.


==831460==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 32816 byte(s) in 1 object(s) allocated from:
    0x7efd2d90de8f in __interceptor_malloc ../../../../src/libsanitizer/asan/asan_malloc_linux.cpp:145
    0x7efd2d2df636 in __alloc_dir ../sysdeps/posix/opendir.c:118

SUMMARY: AddressSanitizer: 32816 byte(s) leaked in 1 allocation(s).

src/util.cc

@@ -2140,6 +2140,7 @@ std::vector<int> read_all_proc_fds(pid_t tid)
   while (struct dirent *dir = readdir(fddir)) {
     ret.push_back(atoi(dir->d_name));
   }
+  closedir(fddir);
   return ret;
 }