bpf: Remove redundant slash

The trailing slash of LIBBPF_SRCS is redundant, remove it. Also inline
it as its only used in LIBBPF_INCLUDE.
Signed-off-by: Yuntao Wang <ytcoode@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220305161013.361646-1-ytcoode@gmail.com

kernel/bpf/preload/Makefile

 # SPDX-License-Identifier: GPL-2.0
 
-LIBBPF_SRCS = $(srctree)/tools/lib/bpf/
-LIBBPF_INCLUDE = $(LIBBPF_SRCS)/..
+LIBBPF_INCLUDE = $(srctree)/tools/lib
 
 obj-$(CONFIG_BPF_PRELOAD_UMD) += bpf_preload.o
-CFLAGS_bpf_preload_kern.o += -I $(LIBBPF_INCLUDE)
+CFLAGS_bpf_preload_kern.o += -I$(LIBBPF_INCLUDE)
 bpf_preload-objs += bpf_preload_kern.o