mirror: Set DESTDIR to empty when building libbpf

[dylanorz]

If a value is assigned to the variable DESTDIR when compiling bpftool, this variable will be passed to libbpf's Makefile.

In this case, libbpf's header files will be installed in an unexpected directory, causing bpftool to be unable to find the header files.

when run this command

OUTPUT=/root/output_3rd/bpftool_build/ DESTDIR=/root/output_3rd/bpftool_root/ V=1 make -j8 install

Part of the output is as follows, we can see that the path is wrong

make[1]: 进入目录“/root/bpftool/libbpf/src”
mkdir -p /root/output_3rd/bpftool_build/libbpf/staticobjs
if [ ! -d '/root/output_3rd/bpftool_root//root/output_3rd/bpftool_build/libbpf/include/bpf' ]; then install -d -m 755 '/root/output_3rd/bpftool_root//root/output_3rd/bpftool_build/libbpf/include/bpf'; fi;
cc -I. -I../include -I../include/uapi -g -O2 -Werror -Wall -std=gnu89 -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Wno-unknown-warning-option -Wno-format-overflow   -c bpf.c -o /root/output_3rd/bpftool_build/libbpf/staticobjs/bpf.o

[dylanorz]

I just have a request, could you please move DESTDIR to the same locations and order (with regards to PREFIX) as they appear in the Makefile from the kernel repo? The two Makefile are mostly similar, and keeping them as close as possible helps with sync-ups.

I've modified this pr as per your suggestion