layers: remove assumption that there's a glslang source tree alongside

We have an in-tree spirv.h now, just use that.
diff --git a/layers/shader_checker.cpp b/layers/shader_checker.cpp
index bd9a62e..e56ad9d 100644
--- a/layers/shader_checker.cpp
+++ b/layers/shader_checker.cpp
@@ -38,7 +38,7 @@
 // being used:
 #include "loader_platform.h"
 
-#include "SPIRV/spirv.h"
+#include "spirv/spirv.h"
 
 
 static std::unordered_map<void *, VkLayerDispatchTable *> tableMap;