Move install directory for SPIRV/ folder.

Currently the SPIRV/ folder will get installed into the include
directory. This folder is part of GLSLang, so it makes more sense under
glslang/SPIRV.

Currently, GLSLang will install a SPIRV/ folder while spirv-headers will
install a spirv/ folder. This is confusing and will cause issues on a
case sensitive filesystem if both are installed at the same time.

SPIRV/CMakeLists.txt

@@ -87,5 +87,6 @@ if(ENABLE_GLSLANG_INSTALL)
                 ARCHIVE DESTINATION ${CMAKE_INSTALL_LIBDIR})
     endif()
 
-    install(FILES ${HEADERS} ${SPVREMAP_HEADERS} DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/SPIRV/)
+    install(FILES ${HEADERS} ${SPVREMAP_HEADERS}
+            DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/glslang/SPIRV/)
 endif(ENABLE_GLSLANG_INSTALL)