samples/bpf: fix in-source build of samples with clang

in-source build of 'make samples/bpf/' was incorrectly
using default compiler instead of invoking clang/llvm.
out-of-source build was ok.

Fixes: a8085782 ("samples: bpf: trivial eBPF program in C")
Signed-off-by: Brenden Blanco <bblanco@plumgrid.com>
Signed-off-by: Alexei Starovoitov <ast@plumgrid.com>
Signed-off-by: David S. Miller <davem@davemloft.net>