bpf: Fix libbpf_version.h build dependency.

Auto generated libbpf_version.h could be causing build issue like:
$ make
CC [U]  kernel/bpf/preload/iterators/iterators.o
In file included from ../tools/lib/bpf/libbpf.h:20,
                 from ../kernel/bpf/preload/iterators/iterators.c:10:
../tools/lib/bpf/libbpf_common.h:13:10: fatal error: libbpf_version.h: No such file or directory
   13 | #include "libbpf_version.h"
      |          ^~~~~~~~~~~~~~~~~~
compilation terminated.
make[4]: *** [../scripts/Makefile.userprogs:43: kernel/bpf/preload/iterators/iterators.o] Error 1

Fix it by force rebuilding libbpf.

Fixes: 0b46b75 ("libbpf: Add LIBBPF_DEPRECATED_SINCE macro for scheduling API deprecations")
Suggested-by: Andrii Nakryiko <[email protected]>
Signed-off-by: Alexei Starovoitov <[email protected]>

Author: Alexei Starovoitov

kernel/bpf/preload/Makefile

@@ -7,7 +7,7 @@ LIBBPF_OUT = $(abspath $(obj))
 # Although not in use by libbpf's Makefile, set $(O) so that the "dummy" test
 # in tools/scripts/Makefile.include always succeeds when building the kernel
 # with $(O) pointing to a relative path, as in "make O=build bindeb-pkg".
-$(LIBBPF_A):
+$(LIBBPF_A): FORCE
 	$(Q)$(MAKE) -C $(LIBBPF_SRCS) O=$(LIBBPF_OUT)/ OUTPUT=$(LIBBPF_OUT)/ $(LIBBPF_OUT)/libbpf.a
 
 userccflags += -I$(LIBBPF_OUT) -I $(srctree)/tools/include/ \