FindX11: Change find_path search string for Xaw

As far as I can tell, X11/Xaw/Intrinsic.h has never existed, resulting
in FindX11 always failing to find Xaw.

Modules/FindX11.cmake

diff --git a/Modules/FindX11.cmake b/Modules/FindX11.cmake
index 6f6483a..aaee24c 100644
--- a/Modules/FindX11.cmake
+++ b/Modules/FindX11.cmake
@@ -135,7 +135,7 @@
   find_path(X11_Xaccessrules_INCLUDE_PATH X11/extensions/XKBrules.h  ${X11_INC_SEARCH_PATH})
   find_path(X11_Xaccessstr_INCLUDE_PATH X11/extensions/XKBstr.h      ${X11_INC_SEARCH_PATH})
   find_path(X11_Xau_INCLUDE_PATH X11/Xauth.h                         ${X11_INC_SEARCH_PATH})
-  find_path(X11_Xaw_INCLUDE_PATH X11/Xaw/Intrinsic.h                 ${X11_INC_SEARCH_PATH})
+  find_path(X11_Xaw_INCLUDE_PATH X11/Xaw/Box.h                       ${X11_INC_SEARCH_PATH})
   find_path(X11_xcb_INCLUDE_PATH xcb/xcb.h                           ${X11_INC_SEARCH_PATH})
   find_path(X11_X11_xcb_INCLUDE_PATH X11/Xlib-xcb.h                  ${X11_INC_SEARCH_PATH})
   find_path(X11_xcb_cursor_INCLUDE_PATH xcb/xcb_cursor.h             ${X11_INC_SEARCH_PATH})