[gdb/testsuite] Fix clone-new-thread-event.c with glibc 2.30

[gdb/testsuite] Fix clone-new-thread-event.c with glibc 2.30

Starting glibc 2.30, unistd.h declares gettid (for _GNU_SOURCE).

This clashes with a static gettid in test source
clone-new-thread-event.c:
...
gdb compile failed, gdb.threads/clone-new-thread-event.c:46:1: error: \
  static declaration of 'gettid' follows non-static declaration
   46 | gettid (void)
      | ^~~~~~
In file included from /usr/include/unistd.h:1170,
                 from gdb.threads/clone-new-thread-event.c:27:
/usr/include/bits/unistd_ext.h:34:16: note: previous declaration of 'gettid' \
  was here
   34 | extern __pid_t gettid (void) __THROW;
      |                ^~~~~~
...

Fix this by renaming the static gettid to local_gettid.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-07-14  Tom de Vries  <tdevries@suse.de>

	* gdb.threads/clone-new-thread-event.c (gettid): Rename to ...
	(local_gettid): ... this.
	(fn): Update.