emacs: fix notmuch-search-line-faces defcustom

In commit 2a7b11b064233afc4feead876fa396e3c18a6b91 the default value
for notmuch-search-line-faces was changed so that it didn't match the
specification in the corresponding defcustom. This meant that it was
difficult for the user to customize this variable as they got a type
mismatch error.

Note anyone who had already customised this variable would not see
this bug as their customisation would match the defcustom.