Fix CMake policy CMP0175 warning in ADD_CUSTOM_COMMAND

Fix CMake policy CMP0175 warning in ADD_CUSTOM_COMMAND

Add explicit POST_BUILD parameter to ADD_CUSTOM_COMMAND at line 227
to comply with CMake policy CMP0175. This eliminates six repeated
warnings during CMake configuration on modern CMake versions.

The command generates shader headers after the BuildShadersHeader
target is built, so POST_BUILD is the appropriate timing.