selftests/bpf: Fix test_cpp compilation failure with clang

With clang compiler:
  make -j60 LLVM=1 LLVM_IAS=1  <=== compile kernel
  make -j60 -C tools/testing/selftests/bpf LLVM=1 LLVM_IAS=1
the test_cpp build failed due to the failure:
  warning: treating 'c-header' input as 'c++-header' when in C++ mode, this behavior is deprecated [-Wdeprecated]
  clang-13: error: cannot specify -o when generating multiple output files

test_cpp compilation flag looks like:
  clang++ -g -Og -rdynamic -Wall -I<...> ... \
  -Dbpf_prog_load=bpf_prog_test_load -Dbpf_load_program=bpf_test_load_program \
  test_cpp.cpp <...>/test_core_extern.skel.h <...>/libbpf.a <...>/test_stub.o \
  -lcap -lelf -lz -lrt -lpthread -o <...>/test_cpp

The clang++ compiler complains the header file in the command line and
also failed the compilation due to this.
Let us remove the header file from the command line which is not intended
any way, and this fixed the compilation problem.

Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210413153424.3028986-1-yhs@fb.com