chore: git ignore gdb.txt file

With `set logging enabled on`, gdb prints the console to a file in the
cwd named `gdb.txt`

Change-Id: Ia1135d89134f0598ad3f288358e928ac70e5a749
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/201584
Reviewed-by: Michael Stahl <michael.stahl@collabora.com>
Tested-by: Jenkins

.gitignore

@@ -165,6 +165,7 @@ LibreOffice.VC.VC.opendb
 /.gdbinit
 /.gdb_history
 gdbtrace.log
+gdb.txt
 
 # Codelite IDE specific
 *.project