build: Fix error message with -Denable-vala without vala found
Fixes ERROR: Unknown variable "vala_options" to ERROR: Problem encountered: you need vala >= 0.14.1 installed to use vala plugins by correcting a typo.
Fixes ERROR: Unknown variable "vala_options" to ERROR: Problem encountered: you need vala >= 0.14.1 installed to use vala plugins by correcting a typo.