[PATCH bpf-next] selftests/bpf: suppress readelf stderr when probing for BTF support

[Stanislav Fomichev <sdf@google.com>]

Before:
$ make -s -C tools/testing/selftests/bpf
readelf: Error: Missing knowledge of 32-bit reloc types used in DWARF
sections of machine number 247
readelf: Warning: unable to apply unsupported reloc type 10 to section
.debug_info
readelf: Warning: unable to apply unsupported reloc type 1 to section
.debug_info
readelf: Warning: unable to apply unsupported reloc type 10 to section
.debug_info

After:
$ make -s -C tools/testing/selftests/bpf

Signed-off-by: Stanislav Fomichev <sdf@google.com>
---
 tools/testing/selftests/bpf/Makefile | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/testing/selftests/bpf/Makefile b/tools/testing/selftests/bpf/Makefile
index 70229de510f5..ff3cab5525c1 100644
--- a/tools/testing/selftests/bpf/Makefile
+++ b/tools/testing/selftests/bpf/Makefile
@@ -131,7 +131,7 @@ BTF_PAHOLE_PROBE := $(shell $(BTF_PAHOLE) --help 2>&1 | grep BTF)
 BTF_OBJCOPY_PROBE := $(shell $(LLVM_OBJCOPY) --help 2>&1 | grep -i 'usage.*llvm')
 BTF_LLVM_PROBE := $(shell echo "int main() { return 0; }" | \
 			  $(CLANG) -target bpf -O2 -g -c -x c - -o ./llvm_btf_verify.o; \
-			  readelf -S ./llvm_btf_verify.o | grep BTF; \
+			  readelf -S ./llvm_btf_verify.o 2>/dev/null | grep BTF; \
 			  /bin/rm -f ./llvm_btf_verify.o)
 
 ifneq ($(BTF_LLVM_PROBE),)
-- 
2.20.1.321.g9e740568ce-goog