package/spirv-headers: needs C++

Fix the following build failure raised since the addition of the package
in commit 0a01085abeb7d8a097cb68b9f7f4faec7711543c:

CMake Error at /home/buildroot/autobuild/instance-3/output-1/host/share/cmake-3.27/Modules/CMakeTestCXXCompiler.cmake:60 (message):
  The C++ compiler

    "/usr/bin/c++"

  is not able to compile a simple test program.

Fixes:
 - http://autobuild.buildroot.org/results/4b94edf6dee03e74ff53939aa228069cc6ba4292

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
[yann.morin.1998@free.fr: propagate to spirv-tools]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

package/spirv-headers/Config.in

@@ -1,6 +1,10 @@
 config BR2_PACKAGE_SPIRV_HEADERS
 	bool "spirv-headers"
+	depends on BR2_INSTALL_LIBSTDCPP
 	help
 	  Machine-readable files for the SPIR-V Registry
 
 	  https://www.khronos.org/registry/spir-v
+
+comment "spirv-headers needs a toolchain w/ C++"
+	depends on !BR2_INSTALL_LIBSTDCPP

package/spirv-tools/Config.in

@@ -1,8 +1,12 @@
 config BR2_PACKAGE_SPIRV_TOOLS
 	bool "spirv-tools"
+	depends on BR2_INSTALL_LIBSTDCPP # spirv-headers
 	select BR2_PACKAGE_SPIRV_HEADERS
 	help
 	  The SPIR-V Tools project provides an API and commands for
 	  processing SPIR-V modules.
 
 	  https://github.com/KhronosGroup/SPIRV-Tools
+
+comment "spirv-tools needs a toolchain w/ C++"
+	depends on !BR2_INSTALL_LIBSTDCPP