[PATCH bpf] libbpf: fix version identification on busybox

[Ivan Khoronzhuk]

It's very often for embedded to have stripped version of sort in
busybox, when no -V option present. It breaks build natively on target
board causing recursive loop.

BusyBox v1.24.1 (2019-04-06 04:09:16 UTC) multi-call binary. \
Usage: sort [-nrugMcszbdfimSTokt] [-o FILE] [-k \
start[.offset][opts][,end[.offset][opts]] [-t CHAR] [FILE]...

Lets modify command a little to avoid -V option.

Fixes: dadb81d0afe732 ("libbpf: make libbpf.map source of truth for libbpf version")

Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@xxxxxxxxxx>
---

Based on bpf/master

 tools/lib/bpf/Makefile | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/lib/bpf/Makefile b/tools/lib/bpf/Makefile
index c6f94cffe06e..a12490ad6215 100644
--- a/tools/lib/bpf/Makefile
+++ b/tools/lib/bpf/Makefile
@@ -3,7 +3,7 @@
 
 LIBBPF_VERSION := $(shell \
 	grep -oE '^LIBBPF_([0-9.]+)' libbpf.map | \
-	sort -rV | head -n1 | cut -d'_' -f2)
+	cut -d'_' -f2 | sort -r | head -n1)
 LIBBPF_MAJOR_VERSION := $(firstword $(subst ., ,$(LIBBPF_VERSION)))
 
 MAKEFLAGS += --no-print-directory
-- 
2.17.1