graphics/libraw: link against libomp with OPENMP=on

Since 07fb2d5 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@)

graphics/libraw/Makefile

@@ -2,7 +2,7 @@
 
 PORTNAME=	libraw
 PORTVERSION=	0.20.2
-PORTREVISION=	1
+PORTREVISION=	2
 CATEGORIES=	graphics
 MASTER_SITES=	http://www.libraw.org/data/
 DISTNAME=	LibRaw-${PORTVERSION}
@@ -38,6 +38,7 @@ LCMS2_LIB_DEPENDS=			liblcms2.so:graphics/lcms2
 
 OPENMP_DESC=				OpenMP support (implies GCC)
 OPENMP_CONFIGURE_ENABLE=		openmp
+OPENMP_LDFLAGS+=			-lomp
 
 RPIRAW_DESC=				Raspberry Pi RAW+JPEG support
 RPIRAW_CPPFLAGS=			-DUSE_6BY9RPI