guest_configs: add missing trailing newline to 5.10 configs

The 5.10 guest configs were missing a trailing newline at the end. This
led to issues in our guest kernel build process, where the last line of
the 5.10 kernels `CONFIG_MICROVM=y` would be combined with the first
line of the CI config fragment `CONFIG_IKCONFIG=y`, leading to a
combined line of `CONFIG_MICROVM=yCONFIG_IKCONFIG=y`. During the
compilation, this line and it's dependents are removed.

The immediate impact of this was that the currently loaded config was
not available in the guest via CONFIG_IKCONFIG (in-kernel config). The
concrete downstream issue we encountered was that the spectre-meltdown
checker test started failing. Specifically, an older version of the
script checked the kernel's retpoline status ONLY via the loaded
Kconfig, so without it available the script could not verify whether the
guest was patched for spectre v2.

Signed-off-by: James Curtis <jxcurtis@amazon.co.uk>