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 1221135289306333d11db25ab20cbbd21ceec630)

Co-authored-by: Florian Bruhin <me@the-compiler.org>
diff --git a/Misc/gdbinit b/Misc/gdbinit
index 45e79fc..e8f62ba 100644
--- a/Misc/gdbinit
+++ b/Misc/gdbinit
@@ -160,7 +160,7 @@
   Print the entire Python call stack - verbose mode
 end
 
-def pu
+define pu
   set $uni = $arg0
   set $i = 0
   while (*$uni && $i++<100)