graphics/libraw: link against libomp with OPENMP=on

Since 07fb2d5e9d00 consumers - most notably shotwell - crashed hard
on RAW file operations when libraw was built with OPENMP=on:

ld-elf.so.1: /usr/local/lib/libraw.so.20: Undefined symbol "__kmpc_fork_call"

adding -lomp to libraw's LDFLAGS (via OPENMP_LDFLAGS, so it stays
conditional) fixes the issue.

PR: 259904
Approved by: maintainer-timeout (jhale@)