[xembedsniproxy] Fix check whether to use XTest

Because of C++'s operator precedence, '!' logically negated
all_event_masks only instead of the whole expression.
This resulted in the condition always being false and XTest never being
used.

Adding a pair of brackets fixes it.

BUG: 362941
BUG: 375017
FIXED-IN: 5.10.1
Differential Revision: https://phabricator.kde.org/D6048