samples/bpf: Pass TPROGS_USER_CFLAGS to libbpf makefile

Before commit [1], the value of a variable TPROGS_USER_CFLAGS was
passed to libbpf make command as a part of EXTRA_CFLAGS.
This commit makes sure that the value of TPROGS_USER_CFLAGS is still
passed to libbpf make command, in order to maintain backwards build
scripts compatibility.

[1] commit 5a6ea70 ("samples/bpf: Remove unnecessary -I flags from libbpf EXTRA_CFLAGS")

Fixes: 5a6ea70 ("samples/bpf: Remove unnecessary -I flags from libbpf EXTRA_CFLAGS")
Suggested-by: Viktor Malik <[email protected]>
Signed-off-by: Eduard Zingerman <[email protected]>
Signed-off-by: Andrii Nakryiko <[email protected]>
Acked-by: Viktor Malik <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]

Author: eddyz87

samples/bpf/Makefile

@@ -123,7 +123,7 @@ always-y += ibumad_kern.o
 always-y += hbm_out_kern.o
 always-y += hbm_edt_kern.o
 
-TPROGS_CFLAGS = $(TPROGS_USER_CFLAGS)
+COMMON_CFLAGS = $(TPROGS_USER_CFLAGS)
 TPROGS_LDFLAGS = $(TPROGS_USER_LDFLAGS)
 
 ifeq ($(ARCH), arm)