Add -Wno-declaration-after-statement to KERNELCPPFLAGS

Disable the warnings regarding ISO C90 forbidding mixed
declarations and code. While this functionality was
introduced as part of C99 gcc does allow this in C90
mode as an extension.

https://gcc.gnu.org/onlinedocs/gcc/Mixed-Declarations.html#Mixed-Declarations

Allowing this usage helps minimize the changes required
when porting patches from OpenZFS. The downside here is
that this functionality is specific to gcc.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #5686