bpf: preload: Add MODULE_DESCRIPTION

Modpost complains when extra warnings are enabled:

WARNING: modpost: missing MODULE_DESCRIPTION() in kernel/bpf/preload/bpf_preload.o

Add a description from the Kconfig help text.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20250310134920.4123633-1-arnd@kernel.org

----
Not sure if that description actually fits what the module does. If not,
please add a different description instead.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/preload/bpf_preload_kern.c

@@ -90,3 +90,4 @@ static void __exit fini(void)
 late_initcall(load);
 module_exit(fini);
 MODULE_LICENSE("GPL");
+MODULE_DESCRIPTION("Embedded BPF programs for introspection in bpffs");