package/opencl-clhpp: add dependency on libopencl

Even though opencl-clhpp's build does not need libopencl, as it's just
made of headers, any user of opencl-clhpp will assume libopencl is
already installed.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/opencl-clhpp/opencl-clhpp.mk

@@ -8,6 +8,7 @@ OPENCL_CLHPP_VERSION = 2.0.16
 OPENCL_CLHPP_SITE = $(call github,KhronosGroup,OpenCL-CLHPP,v$(OPENCL_CLHPP_VERSION))
 OPENCL_CLHPP_LICENSE = Apache-2.0
 OPENCL_CLHPP_LICENSE_FILES = LICENSE.txt
+OPENCL_CLHPP_DEPENDENCIES = libopencl
 OPENCL_CLHPP_INSTALL_STAGING = YES
 
 # OpenCL_CLHPP is a header-only library