samples: kprobes: add missing MODULE_DESCRIPTION() macros

samples: kprobes: add missing MODULE_DESCRIPTION() macros

make allmodconfig && make W=1 C=1 reports:
WARNING: modpost: missing MODULE_DESCRIPTION() in samples/kprobes/kprobe_example.o
WARNING: modpost: missing MODULE_DESCRIPTION() in samples/kprobes/kretprobe_example.o

Add the missing invocations of the MODULE_DESCRIPTION() macro.

Link: https://lore.kernel.org/all/20240601-md-samples-kprobes-v1-1-b6a772353893@quicinc.com/
Signed-off-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>