MAINTAINERS: llext: add tests/misc/llext-edk/ to the llext area

The LLEXT Extension Development Kit, mostly implemented in
'cmake/llext-edk.cmake', is already part of the 'llext' area of the
MAINTAINERS file, but its test is currently unowned.

Add the missing directory to the 'llext' area.

Signed-off-by: Luca Burelli <l.burelli@arduino.cc>