[lumped builds] Add include guards (#pragma once) to header files that did not have any.

The codebase seems to use both “#pragma once“ approach, and “#ifndef / #define” approach,
so I picked pragma once as that one is less typing & less brittle.

When glslang is built with some other build system and lumped/unity builds are used,
without the include guards some headers would get included multiple times, leading to duplicate
declaration errors.

glslang/MachineIndependent/ScanContext.h

@@ -38,6 +38,8 @@
 // sits between the preprocessor scanner and parser.
 //
 
+#pragma once
+
 #include "ParseHelper.h"
 
 namespace glslang {