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.

gdb/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2020-11-12  Gary Benson <gbenson@redhat.com>
+
+	* gdb.threads/tls-so_extern_main.c (tls_ptr): Add missing return
+	statement.
+
 2020-11-11  Simon Marchi  <simon.marchi@efficios.com>
 
 	* gdb.base/continue-after-aborted-step-over.exp: Add "breakpoint

gdb/testsuite/gdb.threads/tls-so_extern_main.c

@@ -25,6 +25,8 @@ tls_ptr (void *p)
 {
    so_extern = &so_extern;
    so_extern2 = &so_extern2; /* break here to check result */
+
+   return NULL;
 }
 
 int