Fix build with musl

Fixes the following build failure:
In file included from ../common/common-cpu.c:32:
../common/common-cpu.h:44:9: error: unknown type name 'cpu_set_t'
   44 |         cpu_set_t *online;
      |         ^~~~~~~~~
../common/common-cpu.h:45:9: error: unknown type name 'cpu_set_t'
   45 |         cpu_set_t *to_keep;
      |         ^~~~~~~~~

Signed-off-by: Kostadin Shishmanov <kocelfc@tutanota.com>

common/common-cpu.h

@@ -31,6 +31,8 @@ POSSIBILITY OF SUCH DAMAGE.
 
 #pragma once
 
+#define _GNU_SOURCE
+
 #include <sched.h>
 #include <stdlib.h>