libublksrv: define _GNU_SOURCE only if it is not already defined

ublksrv needs _GNU_SOURCE defined for using cpu_set_t. While
integrating with external build system like AOSP, these are defined
using CFLAGS (-D_GNU_SOURCE). This causes _GNU_SOURCE redefined build
errors so let's wrap this definition under ifndef which will preserve
original intention while making the integration easier.

Signed-off-by: Sandeep Dhavale <[email protected]>

lib/ublksrv_aio.c

@@ -1,6 +1,9 @@
 // SPDX-License-Identifier: MIT or LGPL-2.1-only
 
+#ifndef _GNU_SOURCE
 #define _GNU_SOURCE
+#endif
+
 #include "ublksrv_priv.h"
 
 #define aio_log ublk_log