package/open62541: add patch to allow building without a C++ compiler

package/open62541: add patch to allow building without a C++ compiler

This patch was intended to be added in commit
b36ea68b5ad0f89ffd92cac3f91654e180683b1c ("package/open62541: new
package") but was missed, causing open62541 to not build on
configurations that lack a C++ compiler. This patch removes the need
for a C++ compiler by properly declaring the CMake project.

Fixes:

  http://autobuild.buildroot.net/results/86ca6a5a01ecfc7030c6be0da81924436b41d057/

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