graphics/nomacs: Fix build when RAW option turned off

The opencv-core dependency is not limited to the RAW option. The build
fails in a clean jail if the RAW option is turned off due to the missing
opencv dependency. Change it to an unconditional LIB_DEPENDS to fix
the build in the RAW-off configuration.

PR: 202497
Reported by: serpent7776 (gmail)
Approved by: maintainer timeout / just-fix-it