Fix proc_show_touches warning

In practice, proc_show_touches may not be used uninitialized, since it
checks the flag options->show_touches, but the compiler can't know that,
so initialize it to avoid the warning.

app/src/scrcpy.c

@@ -125,7 +125,7 @@ SDL_bool scrcpy(const struct scrcpy_options *options) {
         return SDL_FALSE;
     }
 
-    process_t proc_show_touches;
+    process_t proc_show_touches = PROCESS_NONE;
     SDL_bool show_touches_waited;
     if (options->show_touches) {
         LOGI("Enable show_touches");