compel: add missing header required by musl

This fix compilation issue regarding undeclared NULL and memcpy when using musl
on ARM.
Signed-off-by: Pierre-Olivier Mercier <nemunaire@nemunai.re>

compel/arch/aarch64/src/lib/infect.c

+#include <stdlib.h>
 #include <sys/ptrace.h>
 #include <sys/types.h>
 #include <sys/uio.h>

compel/arch/arm/src/lib/infect.c

+#include <stdlib.h>
 #include <sys/ptrace.h>
 #include <sys/types.h>
 #include <compel/plugins/std/syscall-codes.h>