[gdb/testsuite] Fix gdb.base/print-symbol-loading.exp on ubuntu 22.04.1

[gdb/testsuite] Fix gdb.base/print-symbol-loading.exp on ubuntu 22.04.1

On ubuntu 22.04.1 x86_64, I run into:
...
(gdb) PASS: gdb.base/print-symbol-loading.exp: shlib off: \
  set print symbol-loading off
sharedlibrary .*^M
Symbols already loaded for /lib/x86_64-linux-gnu/libc.so.6^M
Symbols already loaded for /lib/x86_64-linux-gnu/libpthread.so.0^M
(gdb) FAIL: gdb.base/print-symbol-loading.exp: shlib off: load shared-lib
...

The test-case expects the libc.so line, but not the libpthread.so line.

However, we have:
...
$ ldd /lib/x86_64-linux-gnu/libc.so.6
	linux-vdso.so.1 (0x00007ffd7f7e7000)
	libgtk3-nocsd.so.0 => /lib/x86_64-linux-gnu/libgtk3-nocsd.so.0 (0x00007f4468c00000)
	/lib64/ld-linux-x86-64.so.2 (0x00007f4469193000)
	libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007f4468f3e000)
	libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007f4468f39000)
...
so it's not unexpected that libpthread.so is loaded if libc.so is loaded.

Fix this by accepting the libpthread.so line.

Tested on x86_64-linux.

PR testsuite/29919
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29919