ipc: enable _GNU_SOURCE to fix build with musl libc

This fixes compile the compile error:

> ipc.c:154:49: error: 'struct shm_info' has no member named 'used_ids'
>    ipc_submit_g("shm", "segments", NULL, shm_info.used_ids);
>                                                   ^

Fixes collectd#1147

src/ipc.c

@@ -32,6 +32,9 @@
 #include "configfile.h"
 
 #if KERNEL_LINUX
+  /* _GNU_SOURCE is needed for struct shm_info.used_ids on musl libc */
+# define _GNU_SOURCE
+
   /* X/OPEN tells us to use <sys/{types,ipc,sem}.h> for semctl() */
   /* X/OPEN tells us to use <sys/{types,ipc,msg}.h> for msgctl() */
   /* X/OPEN tells us to use <sys/{types,ipc,shm}.h> for shmctl() */