MAINTAINERS: record lib/buildid.c as owned by BPF subsystem

Build ID fetching code originated from ([0]), and is still both owned
and heavily relied upon by BPF subsystem.

Fix the original omission in [0] to record this fact in MAINTAINERS.

  [0] bd7525da ("bpf: Move stack_map_get_build_id into lib")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Song Liu <song@kernel.org>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/r/20240909190426.2229940-1-andrii@kernel.orgSigned-off-by: Alexei Starovoitov <ast@kernel.org>

MAINTAINERS

@@ -4098,6 +4098,7 @@ F:	include/uapi/linux/btf*
 F:	include/uapi/linux/filter.h
 F:	kernel/bpf/
 F:	kernel/trace/bpf_trace.c
+F:	lib/buildid.c
 F:	lib/test_bpf.c
 F:	net/bpf/
 F:	net/core/filter.c
@@ -4218,6 +4219,7 @@ L:	bpf@vger.kernel.org
 S:	Maintained
 F:	kernel/bpf/stackmap.c
 F:	kernel/trace/bpf_trace.c
+F:	lib/buildid.c
 
 BROADCOM ASP 2.0 ETHERNET DRIVER
 M:	Justin Chen <justin.chen@broadcom.com>