build: ensure dependencies are installed for `memmap` target

The `memmap` build system target was recently migrated over to an
independent Poetry project, but its build system target was not updated
to install its dependencies.

Change-Id: If46e2a4609d47467cac07426c1cde65e2e0944cb
Signed-off-by: Chris Kay <chris.kay@arm.com>
1 file changed