Include config.h early for _GNU_SOURCE with musl libc

Include config.h early for _GNU_SOURCE with musl libc

This fixes various compile errors when building with musl libc. For
example:

In file included from start.c:66:0:
monitor.h:38:12: error: 'NAME_MAX' undeclared here (not in a function)
  char name[NAME_MAX+1];
            ^
start.c: In function 'setup_signal_fd':
start.c:202:2: error: implicit declaration of function 'sigfillset' [-Werror=implicit-function-declaration]
  if (sigfillset(&mask) ||
  ^

...

In file included from freezer.c:36:0:
monitor.h:39:12: error: 'NAME_MAX' undeclared here (not in a function)
  char name[NAME_MAX+1];
            ^
...

In file included from cgroup.c:45:0:
conf.h:87:13: error: 'IFNAMSIZ' undeclared here (not in a function)
  char veth1[IFNAMSIZ]; /* needed for deconf */
             ^
cgroup.c: In function 'find_cgroup_subsystems':
cgroup.c:230:3: error: implicit declaration of function 'strdup' [-Werror=implicit-function-declaration]
   (*kernel_subsystems)[kernel_subsystems_count] = strdup(line);
   ^
...

In file included from conf.c:65:0:
conf.h:87:13: error: 'IFNAMSIZ' undeclared here (not in a function)
  char veth1[IFNAMSIZ]; /* needed for deconf */
             ^
In file included from conf.c:66:0:
conf.c: In function 'run_buffer':
log.h:263:9: error: implicit declaration of function 'strsignal' [-Werror=implicit-function-declaration]
  struct lxc_log_locinfo locinfo = LXC_LOG_LOCINFO_INIT;  \
         ^
...

af_unix.c: In function 'lxc_abstract_unix_send_credential':
af_unix.c:208:9: error: variable 'cred' has initializer but incomplete type
  struct ucred cred = {
         ^
af_unix.c:209:3: error: unknown field 'pid' specified in initializer
   .pid = getpid(),
   ^
af_unix.c:209:3: error: excess elements in struct initializer [-Werror]
af_unix.c:209:3: error: (near initialization for 'cred') [-Werror]
af_unix.c:210:3: error: unknown field 'uid' specified in initializer
   .uid = getuid(),
   ^
af_unix.c:210:3: error: excess elements in struct initializer [-Werror]
af_unix.c:210:3: error: (near initialization for 'cred') [-Werror]
af_unix.c:211:3: error: unknown field 'gid' specified in initializer
   .gid = getgid(),
   ^

and more...

Signed-off-by: Natanael Copa <ncopa@alpinelinux.org>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>