build: set -Wno-format-signedness

[vszakats]

Explicitly disable these warnings to allow using -Weverything.

There are around 600 of them across the codebase.

Silencing them has some drawbacks:

• enums (CURLcode mostly) would have to be cast to int to avoid
  different signedness depending on C compiler.
  (llvm/gcc: unsigned, MSVC/clang-cl: signed by default)
• hex masks need casts to unsigned to avoid the warning.
• fixing remaining warnings is annoying without fixing the above.
• without fixing all warnings the option cannot be enabled, to keep
  the codebase warning free.

Ref: #18343 (silenced all warnings, but without the enum cast)
Follow-up to 92f215f #18477