ensure header only libs live in the fggl folder

vendor/header_only/CMakeLists.txt

 target_include_directories( fggl
     PUBLIC
         $<BUILD_INTERFACE:${CMAKE_CURRENT_LIST_DIR}/public>
-        $<INSTALL_INTERFACE:include>
+        $<INSTALL_INTERFACE:include/fggl>
 )
 
 # FIXME should be a cleaner way to do this...
@@ -9,12 +9,12 @@ install(
     FILES
         public/glad/glad.h
     DESTINATION
-        ${CMAKE_INSTALL_INCLUDEDIR}/glad
+        ${CMAKE_INSTALL_INCLUDEDIR}/fggl/glad
 )
 
 install(
         FILES
             public/KHR/khrplatform.h
         DESTINATION
-            ${CMAKE_INSTALL_INCLUDEDIR}/KHR
+            ${CMAKE_INSTALL_INCLUDEDIR}/fggl/KHR
 )