Move openmp to optdepends

It is not required for building VTK, but when building downstream
projects with Clang.

.SRCINFO

@@ -41,7 +41,6 @@ pkgbase = vtk
 	makedepends = nlohmann-json
 	makedepends = opencascade
 	makedepends = openimagedenoise
-	makedepends = openmp
 	makedepends = openmpi
 	makedepends = openvdb
 	makedepends = openvr
@@ -93,6 +92,7 @@ pkgbase = vtk
 	optdepends = netcdf
 	optdepends = opencascade
 	optdepends = openimagedenoise
+	optdepends = openmp: OpenMP support for downstream projects built with Clang
 	optdepends = openmpi: OpenMPI support
 	optdepends = openvdb
 	optdepends = openvr

PKGBUILD

@@ -59,7 +59,6 @@ makedepends=(
   nlohmann-json
   opencascade
   openimagedenoise
-  openmp
   openmpi
   openvdb
   openvr
@@ -109,6 +108,7 @@ optdepends=(
   netcdf
   opencascade
   openimagedenoise
+  'openmp: OpenMP support for downstream projects built with Clang'
   'openmpi: OpenMPI support'
   openvdb
   openvr