build: use GNUInstallDirs install destinations

So as to be compliant with distribution layouts, as recommend by:
https://cmake.org/cmake/help/latest/command/install.html#installing-files

Type: make
Change-Id: Ic46ace4f26aab1aa4902cbd013c40a92c480680d
Signed-off-by: Nick Brown <nickbroon@gmail.com>
diff --git a/src/vpp-api/vapi/CMakeLists.txt b/src/vpp-api/vapi/CMakeLists.txt
index 53034bd..df5d611 100644
--- a/src/vpp-api/vapi/CMakeLists.txt
+++ b/src/vpp-api/vapi/CMakeLists.txt
@@ -34,7 +34,7 @@
     vapi.hpp
     vapi_internal.h
   DESTINATION
-    include/vapi
+    ${CMAKE_INSTALL_INCLUDEDIR}/vapi
   COMPONENT
     vpp-dev
 )