tests: fix bpf ops build error

If removed the previous install by 'rm -rf /usr/include/liburing', the
test build will fail:

make[1]: Entering directory '/root/linux/liburing/test'
     CC helpers.o
mkdir -p output/bpf
     CC output/bpf/nops.bpf.o
mkdir -p output/bpf
     CC output/bpf/cp.bpf.o
In file included from /root/linux/liburing/test/bpf-progs/nops.bpf.c:2:
/root/linux/liburing/test/bpf-progs/../bpf_defs.h:9:10: fatal error: 'liburing/io_uring.h' file not found
    9 | #include "liburing/io_uring.h"
      |          ^~~~~~~~~~~~~~~~~~~~~
1 error generated.
make[1]: *** [Makefile:387: output/bpf/nops.bpf.o] Error 1
make[1]: *** Waiting for unfinished jobs....
In file included from /root/linux/liburing/test/bpf-progs/cp.bpf.c:2:
/root/linux/liburing/test/bpf-progs/../bpf_defs.h:9:10: fatal error: 'liburing/io_uring.h' file not found
    9 | #include "liburing/io_uring.h"
      |          ^~~~~~~~~~~~~~~~~~~~~
1 error generated.
make[1]: *** [Makefile:387: output/bpf/cp.bpf.o] Error 1
make[1]: Leaving directory '/root/linux/liburing/test'
make: *** [Makefile:14: all] Error 2

Add the include option in 'CPPFLAGS' for bpf build.

Fixes: fd8a6e66c739 ("tests: test io_uring bpf ops")
Signed-off-by: Haiyue Wang <haiyuewa@163.com>
Link: https://patch.msgid.link/20260426112732.300165-1-haiyuewa@163.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>