package/poke: fix build with uclibc-ng < 1.0.35

package/poke: fix build with uclibc-ng < 1.0.35

Fix the folllowing build failure with uclibc-ng which is raised since
activation of uclibc-ng in commit
a4dc754178b7c24ed129e7e6f8effd555249c941:

In file included from ./sys/random.h:40,
                 from getrandom.c:22:
/tmp/instance-0/output-1/host/x86_64-buildroot-linux-uclibc/sysroot/usr/include/sys/random.h:27:35: error: unknown type name 'size_t'
   27 | extern int getrandom(void *__buf, size_t count, unsigned int flags)
      |                                   ^~~~~~
/tmp/instance-0/output-1/host/x86_64-buildroot-linux-uclibc/sysroot/usr/include/sys/random.h:8:1: note: 'size_t' is defined in header '<stddef.h>'; did you forget to '#include <stddef.h>'?
    7 | #include <features.h>
  +++ |+#include <stddef.h>
    8 |

Fixes:
 - http://autobuild.buildroot.org/results/30105094e39374ec6d4e47e2fb5e99696f7f7981

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>