libdwfl: clean up non-Linux GNU platforms code

libdwfl: clean up non-Linux GNU platforms code

For non-Linux GNU platforms (like kFreeBSD, Hurd), linux-pid-attach.c
had some stub functions that are not used or exported.  Since gcc-6,
having these caused compiler errors due to -Wall -Werror:

linux-pid-attach.c:479:36: error: 'pid_thread_callbacks' defined but not used [-Werror=unused-const-variable=]

linux-pid-attach.c:474:1: error: 'pid_thread_detach' defined but not used [-Werror=unused-function]
linux-pid-attach.c:461:1: error: 'pid_detach' defined but not used [-Werror=unused-function]
linux-pid-attach.c:452:1: error: 'pid_set_initial_registers' defined but not used [-Werror=unused-function]
linux-pid-attach.c:441:1: error: 'pid_memory_read' defined but not used [-Werror=unused-function]
linux-pid-attach.c:420:1: error: 'pid_getthread' defined but not used [-Werror=unused-function]
linux-pid-attach.c:410:1: error: 'pid_next_thread' defined but not used [-Werror=unused-function]

This part of the source file is guarded by #ifndef __linux__

Signed-off-by: Steven Chamberlain <steven@pyro.eu.org>