od: fix issues with recent format string changes

od: fix issues with recent format string changes

* src/ioblksize.h: Avoid syntax check with redundant idx.h inclusion.
* src/od.c (FMT_BYTES_ALLOCATED): Increase by two to avoid:
  error: '%s' directive writing between 1 and 2 bytes into a region
  of size between 1 and 4 [-Werror=format-overflow=]
(maint): Use %td to print idx_t rather than invalid %jt format.