[Pal/Linux-SGX] Fix warning in sgx_gdb.c

Fix warning of implicit declaration of syscall/pread/pwrite.

Signed-off-by: Isaku Yamahata <isaku.yamahata@gmail.com>

Pal/src/host/Linux-SGX/debugger/sgx_gdb.c

@@ -1,6 +1,7 @@
 /* -*- mode:c; c-file-style:"k&r"; c-basic-offset: 4; tab-width:4; indent-tabs-mode:nil; mode:auto-fill; fill-column:78; -*- */
 /* vim: set ts=4 sw=4 et tw=78 fo=cqt wm=0: */
 
+#define _GNU_SOURCE
 #include <stddef.h>
 #include <unistd.h>
 #include <stdio.h>