oeqa/runtime/stap: increase buffer size

We're seeing random failures in the SystemTap tests in qemuarm on kernel
5.10. This might be related to the buffer between user and kernel space
being too small, so explicitly set the size.

If this cures the problem it should be considered a workaround and not
the solution.

[ YOCTO #14673 ]

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>