alsa-mixer-path-test: Hide unused functions when building with Meson

alsa-mixer-path-test: Hide unused functions when building with Meson

Silences these warnings:

[509/574] Compiling C object 'src/tests/a4ccf2d@@alsa-mixer-path-test@exe/alsa-mixer-path-test.c.o'.
../src/tests/alsa-mixer-path-test.c:24:20: warning: ‘load_makefile’ defined but not used [-Wunused-function]
 static pa_strlist *load_makefile() {
                    ^~~~~~~~~~~~~
../src/tests/alsa-mixer-path-test.c:17:20: warning: ‘get_default_paths_dir’ defined but not used [-Wunused-function]
 static const char *get_default_paths_dir(void) {
                    ^~~~~~~~~~~~~~~~~~~~~