Allow c99 when building ZFS in the kernel tree

Commit 4a5d7f82 enabled building c99 out of the kernel tree.
However, when building as part of the kernel different Makefiles
are used and -std=gnu99 must additionially be added there.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #5919