Fix gdb.threads/tls-so_extern.exp with Clang

Clang fails to compile gdb.threads/tls-so_extern_main.c, giving the
following error:

  /gdbtest/src/gdb/testsuite/gdb.threads/tls-so_extern_main.c:28:1:
    warning: non-void function does not return a value [-Wreturn-type]

This commit adds a return statement to the offending function.

gdb/testsuite/ChangeLog:

	* gdb.threads/tls-so_extern_main.c (tls_ptr): Add missing return
	statement.