Move prctl PR_SET_DUMPABLE into platform.c.

This should make it easier to add additional platform support such as
Solaris (bz#2584).
diff --git a/platform.h b/platform.h
index e687c99..e97ecd9 100644
--- a/platform.h
+++ b/platform.h
@@ -31,6 +31,7 @@
 char *platform_get_krb5_client(const char *);
 char *platform_krb5_get_principal_name(const char *);
 int platform_sys_dir_uid(uid_t);
+void platform_disable_tracing(int);
 
 /* in platform-pledge.c */
 void platform_pledge_agent(void);