Fix mis-merge.

In propagating the requirements for opm-benchmarks to the other
modules I accidentally removed the 'opm-parser' prerequisite (see
commit 86439d6 for details).  This commit restores that prerequisite.

cmake/Modules/opm-core-prereqs.cmake

@@ -25,6 +25,8 @@ set (opm-core_DEPS
 	"SuperLU"
 	# xml processing (for config parsing)
 	"TinyXML"
+	#Parser library
+	"opm-parser REQUIRED"
 	# Ensembles-based Reservoir Tools (ERT)
 	"ERT"
 	# DUNE dependency