[gdb/testsuite] Fix gdb_unbuffer_output return-type

[gdb/testsuite] Fix gdb_unbuffer_output return-type

When running test-case gdb.base/shlib-call.exp with clang, we get:
...
gdb compile failed, In file included from shmain.c:6:
unbuffer_output.c:39:1: warning:
      control reaches end of non-void function [-Wreturn-type]
}
^
1 warning generated.
...

Fix this by changing the return-type to void.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-05-06  Tom de Vries  <tdevries@suse.de>

	* lib/unbuffer_output.c (gdb_unbuffer_output): Change return type to
	void.