Are you sure it's a curly brace '{', and not a square bracket '['? The square bracket is just an alternative (and quite common) syntax for invoking the command test (1). According to the man page, '-x' checks if a file is executable, '-a' means 'and', '-e' checks if a file exists.

So in other words, if the command was actually something like

if [ -x /usr/bin/find -a -e /lib/modules/$RELEASE/modules.dep ]

if ( ( /usr/bin/find is executable) and ( /lib/modules/$RELEASE/modules.dep exists ) )