package/vulkan-headers: needs C++

Since Buildroot commit [1] "package/vulkan-{headers,loader,tools}:
bump to 1.4.317 release", the vulkan-headers package is failing at
configuration time with toolchains without C++ support, with error:

    The C++ compiler
      "/bin/false"
    is not able to compile a simple test program.

This is due to upstream commit [2], which added CXX to the CMake
project file.

This commit fixes the issue by adding the C++ dependency.

Fixes:
https://autobuild.buildroot.org/results/10f63398940d52be9a8ee306daf4203ceca25eb8

[1] https://gitlab.com/buildroot.org/buildroot/-/commit/52664093dee0f280db191c391167848dc5b09745
[2] https://github.com/KhronosGroup/Vulkan-Headers/commit/85256c712ce9ee9755ae74faacc3e6a1d2c5eddf

Signed-off-by: Julien Olivain <ju.o@free.fr>

package/vulkan-headers/Config.in

@@ -1,7 +1,11 @@
 config BR2_PACKAGE_VULKAN_HEADERS
 	bool "vulkan-headers"
+	depends on BR2_INSTALL_LIBSTDCPP
 	help
 	  The Khronos official Vulkan header files and API
 	  registry.
 
 	  https://github.com/KhronosGroup/Vulkan-Headers
+
+comment "vulkan-headers needs a toolchain w/ C++"
+	depends on !BR2_INSTALL_LIBSTDCPP