gdbinit: Use proper define syntax (GH-19557)

Using `def` rather than `define` results in:

    Ambiguous command "def pu": define, define-prefix.

Automerge-Triggered-By: @csabella
(cherry picked from commit 1221135289)

Co-authored-by: Florian Bruhin <me@the-compiler.org>

Misc/gdbinit

@@ -160,7 +160,7 @@ document pystackv
   Print the entire Python call stack - verbose mode
 end
 
-def pu
+define pu
   set $uni = $arg0
   set $i = 0
   while (*$uni && $i++<100)