Do not use `-Wformat-signedness` when compiling with clang (clang doesn't know this flag)

[qmonnet]

Commit 4c56992b53cc125f6939a971f01d4705996f5e0f introduced the use of warning -Wformat-signedness when compiling bpftool, but this warning seems to be unknown to clang, which now produces the following output when compiling bpftool:

[...]
warning: warning: unknown warning option '-Wformat-signedness' [-Wunknown-warning-option]
unknown warning option '-Wformat-signedness' [-Wunknown-warning-option]
warning: unknown warning option '-Wformat-signedness' [-Wunknown-warning-option]
warning: unknown warning option '-Wformat-signedness' [-Wunknown-warning-option]
warning: unknown warning option '-Wformat-signedness' [-Wunknown-warning-option]
warning: unknown warning option '-Wformat-signedness' [-Wunknown-warning-option]
warning: unknown warning option '-Wformat-signedness' [-Wunknown-warning-option]
warning: unknown warning option '-Wformat-signedness' [-Wunknown-warning-option]
warning: unknown warning option '-Wformat-signedness' [-Wunknown-warning-option]
warning: unknown warning option '-Wformat-signedness' [-Wunknown-warning-option]
warning: unknown warning option '-Wformat-signedness' [-Wunknown-warning-option]
warning: unknown warning option '-Wformat-signedness' [-Wunknown-warning-option]
warning: unknown warning option '-Wformat-signedness' [-Wunknown-warning-option]
warning: unknown warning option '-Wformat-signedness' [-Wunknown-warning-option]
warning: unknown warning option '-Wformat-signedness' [-Wunknown-warning-option]
warning: unknown warning option '-Wformat-signedness' [-Wunknown-warning-option]
warning: unknown warning option '-Wformat-signedness' [-Wunknown-warning-option]
[...]

(Example from last PR)

We should not use this warning when compiling with clang. Should be easy enough to fix in the Makefile.

Cc @mrpre, if you feel like looking into it 🙂 I'll fix it at some point, otherwise.