Move platform_disable_tracing into its own file.

Prevents link errors resolving the extern "options" when platform.o
gets linked into ssh-agent when building --with-pam.
diff --git a/Makefile.in b/Makefile.in
index 1a2e743..141608b 100644
--- a/Makefile.in
+++ b/Makefile.in
@@ -92,13 +92,13 @@
 	kex.o kexdh.o kexgex.o kexecdh.o kexc25519.o \
 	kexdhc.o kexgexc.o kexecdhc.o kexc25519c.o \
 	kexdhs.o kexgexs.o kexecdhs.o kexc25519s.o \
-	platform.o platform-pledge.o
+	platform-pledge.o platform-tracing.o
 
 SSHOBJS= ssh.o readconf.o clientloop.o sshtty.o \
 	sshconnect.o sshconnect1.o sshconnect2.o mux.o
 
 SSHDOBJS=sshd.o auth-rhosts.o auth-passwd.o auth-rsa.o auth-rh-rsa.o \
-	audit.o audit-bsm.o audit-linux.o \
+	audit.o audit-bsm.o audit-linux.o platform.o \
 	sshpty.o sshlogin.o servconf.o serverloop.o \
 	auth.o auth1.o auth2.o auth-options.o session.o \
 	auth-chall.o auth2-chall.o groupaccess.o \