Fix compilation of sandbox.[ch] under musl-libc

Patch from jamestk; fix on 0.2.5.1-alpha. Fixes 17347.

changes/bug17347

@@ -0,0 +1,4 @@
+  o Minor bugfixes (compilation):
+    - Fix compilation of sandbox.c with musl-libc.
+      Fixes bug 17347; bugfix on 0.2.5.1-alpha.
+      Patch from 'jamestk'.

src/common/sandbox.c

@@ -48,7 +48,7 @@
 #include <sys/epoll.h>
 #include <sys/prctl.h>
 #include <linux/futex.h>
-#include <bits/signum.h>
+#include <sys/file.h>
 
 #include <stdarg.h>
 #include <seccomp.h>

src/common/sandbox.h

@@ -42,6 +42,9 @@ typedef struct sandbox_cfg_elem sandbox_cfg_t;
 #ifndef __USE_GNU
 #define __USE_GNU
 #endif
+#ifndef _GNU_SOURCE
+#define _GNU_SOURCE
+#endif
 #include <sys/ucontext.h>
 #include <seccomp.h>
 #include <netdb.h>