make the public install interface point to /include

This way clients have to #include <hlslcc/hlslcc.hpp> (or pstdint.h could conflict with previously existing files)

CMakeLists.txt

@@ -44,7 +44,7 @@ target_include_directories(hlslcc
     PUBLIC
     $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/>
     $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>
-    $<INSTALL_INTERFACE:include/hlslcc>
+    $<INSTALL_INTERFACE:include>
     PRIVATE
     $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/src>
     $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/src/cbstring>