[chore](trx-ft8): suppress unused code warnings in external C library

Add compiler flags to suppress C warnings from vendored ft8_lib:
- -Wno-unused-const-variable for db_power_sum array
- -Wno-unused-function for ft8_decode_multi_symbols

These are harmless warnings from external code we don't control.
Suppressing at build system level keeps external code unchanged.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Stanislaw Grams <stanislawgrams@gmail.com>

src/trx-ft8/build.rs

@@ -23,6 +23,8 @@ fn main() {
         .file(format!("{base}/ft8/message.c"))
         .file(format!("{base}/ft8/text.c"))
         .flag_if_supported("-std=c99")
+        .flag_if_supported("-Wno-unused-const-variable")
+        .flag_if_supported("-Wno-unused-function")
         .compile("trx_ft8");
 
     println!("cargo:rustc-link-lib=m");