[fix] xpath, mojeek: fix commit 58d72f26

before commit 58d72f26, category was not set in xpath.py,
so searx/engines/__init__py was setting the category to ['general']

the commit 58d72f26 set the category to [] which is not replaced by searx/engines/__init__.py
consequence: the mojeek engine is hidden in the preferences.

this commit revert the xpath.py change.

close #2368