Plug memory leak in GTK x-display-monitor-attributes-list

* src/frame.c (free_monitors) [USE_GTK]: Define in the GTK case as
  well.

* src/xfns.c (x-display-monitor-attributes-list) [USE_GTK]: Plug
  memory leak.

* src/frame.h (MonitorInfo): Declare name as pointing to const char.
1 job for master in 60 minutes and 2 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
failed #1257
test-all

01:00:02

 
Name Stage Failure
failed
test-all Test The script exceeded the maximum execution time set for the job
make -C test check-expensive
make[1]: Entering directory '/builds/emacs/emacs/test'
rm -f ./*.tmp
make[2]: Entering directory '/builds/emacs/emacs/test'
make[3]: Entering directory '/builds/emacs/emacs/test'
ELC lisp/net/tramp-tests.elc
GEN lisp/net/tramp-tests.log
Pulling docker image gitlab/gitlab-runner-helper:x86_64-f100a208 ...
ERROR: Job failed: execution took longer than 1h0m0s seconds