collectd: fix musl builds (ipc module)

https://github.com/collectd/collectd/commit/3e3848349b753d78a0b1d19648fb394866856bda

    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 #1147

Signed-off-by: Alex Suykov <alex.suykov@gmail.com>

diff --git a/src/ipc.c b/src/ipc.c

index 3763f24..b403847 100644

--- a/src/ipc.c

+++ b/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() */