kbuild, bpf: Correct pahole version that supports distilled base btf feature

pahole commit [0] of supporting distilled base btf feature released on
pahole v1.28 rather than v1.26. So let's correct this.

Signed-off-by: Pu Lehui <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Link: https://git.kernel.org/pub/scm/devel/pahole/pahole.git/commit/?id=c7b1f6a29ba1 [0]
Link: https://lore.kernel.org/bpf/[e-mail]

scripts/Makefile.btf

···
 pahole-flags-$(call test-ge, $(pahole-ver), 126)  = -j$(JOBS) --btf_features=encode_force,var,float,enum64,decl_tag,type_tag,optimized_func,consistent_func,decl_tag_kfuncs
 
 ifneq ($(KBUILD_EXTMOD),)
-module-pahole-flags-$(call test-ge, $(pahole-ver), 126) += --btf_features=distilled_base
+module-pahole-flags-$(call test-ge, $(pahole-ver), 128) += --btf_features=distilled_base
 endif
 
 endif