[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.

SPIRV/GlslangToSpv.h

@@ -32,6 +32,8 @@
 // ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 // POSSIBILITY OF SUCH DAMAGE.
 
+#pragma once
+
 #if _MSC_VER >= 1900
     #pragma warning(disable : 4464) // relative include path contains '..'
 #endif

SPIRV/doc.h

@@ -36,6 +36,8 @@
 // Parameterize the SPIR-V enumerants.
 //
 
+#pragma once
+
 #include "spirv.hpp"
 
 #include <vector>