- Some headers graduated from the blacklist

 - hyperv_timer.h joined the header-test when CONFIG_X86=y

 - nf_tables*.h joined the header-test when CONFIG_NF_TABLES is
   enabled.

 - The entry for nf_tables_offload.h was added to fix build error for
   the combination of CONFIG_NF_TABLES=n and CONFIG_KERNEL_HEADER_TEST=y.

 - The entry for iomap.h was added because this header is supposed to
   be included only when CONFIG_BLOCK=y
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>