[CMake] Fix include path in XRootDConfig.cmake

Headers are installed into the subdirectory xrootd/. This was broken
with commit c6e0e59 ("[CMake] Find only XRootD matching
XRootDConfig.cmake installation path") released with version 5.6.3.

cmake/XRootDConfig.cmake.in

@@ -50,7 +50,7 @@ SET( XROOTD_SSI_FOUND     FALSE )
 
 set_and_check(XRootD_CMAKE_DIR   "${CMAKE_CURRENT_LIST_DIR}")
 set_and_check(XRootD_DATA_DIR    "@PACKAGE_CMAKE_INSTALL_DATADIR@")
-set_and_check(XRootD_INCLUDE_DIR "@PACKAGE_CMAKE_INSTALL_INCLUDEDIR@")
+set_and_check(XRootD_INCLUDE_DIR "@PACKAGE_CMAKE_INSTALL_INCLUDEDIR@/xrootd")
 set_and_check(XRootD_LIB_DIR     "@PACKAGE_CMAKE_INSTALL_LIBDIR@")
 
 set(XRootD_INCLUDE_DIRS "${XRootD_INCLUDE_DIR};${XRootD_INCLUDE_DIR}/private")