fix(omnivoice-codec): make main_impl static to satisfy -Wmissing-declarations

`int main_impl(int, char**)` in tools/omnivoice/tools/omnivoice-codec.cpp
had no prior declaration. Under `-DLLAMA_FATAL_WARNINGS=ON` (CI
3rd-party / ubuntu-24-llguidance) this fails:

  tools/omnivoice/tools/omnivoice-codec.cpp:158:5:
    error: no previous declaration for 'int main_impl(int, char**)'
    [-Werror=missing-declarations]

The sibling tool tools/omnivoice/tools/omnivoice-tts.cpp already marks
its main_impl static (line 411). Match it.

Author: lalalune

tools/omnivoice/tools/omnivoice-codec.cpp

@@ -155,7 +155,7 @@ static int infer_mode(const char * path) {
     return 0;
 }
 
-int main_impl(int argc, char ** argv) {
+static int main_impl(int argc, char ** argv) {
     if (argc <= 1) {
         print_usage(argv[0]);
         return 0;