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/CMakeLists.txt b/layers/CMakeLists.txt
index ff746a0..6988d8d 100644
--- a/layers/CMakeLists.txt
+++ b/layers/CMakeLists.txt
@@ -38,7 +38,6 @@
 include_directories(
 	${CMAKE_CURRENT_SOURCE_DIR}
 	${CMAKE_CURRENT_SOURCE_DIR}/../loader
-	${CMAKE_CURRENT_SOURCE_DIR}/../../glslang
 	${CMAKE_CURRENT_BINARY_DIR}
 )
 
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;