depends: Prefix include path for headers-only systemtap package

depends/toolchain.cmake.in

@@ -163,6 +163,7 @@ if("@usdt_packages@" MATCHES "^[ ]*$")
   set(WITH_USDT OFF CACHE BOOL "")
 else()
   set(WITH_USDT ON CACHE BOOL "")
+  set(USDT_INCLUDE_DIR "${CMAKE_CURRENT_LIST_DIR}/systemtap/include" CACHE PATH "")
 endif()
 
 set(ipc_packages @ipc_packages@)