Fix warnings: static function, {} initializier, un-needed enum forward declaration

src/core/linux/SDL_evdev_kbd.c

@@ -219,7 +219,7 @@ static void kbd_cleanup(void)
     ioctl(kbd->console_fd, KDSKBMODE, kbd->old_kbd_mode);
 }
 
-void
+static void
 SDL_EVDEV_kbd_reraise_signal(int sig)
 {
     raise(sig);

src/core/linux/SDL_threadprio.c

@@ -108,7 +108,9 @@ rtkit_initialize_realtime_thread()
     int nLimit = RLIMIT_RTTIME;
     pid_t nPid = 0; //self
     int nSchedPolicy = sched_getscheduler(nPid) | SCHED_RESET_ON_FORK;
-    struct sched_param schedParam = {};
+    struct sched_param schedParam;
+
+    SDL_zero(schedParam);
 
     // Requirement #1: Set RLIMIT_RTTIME
     err = getrlimit(nLimit, &rlimit);

src/video/wayland/SDL_waylanddyn.h

@@ -42,9 +42,6 @@ struct libdecor_state;
 struct libdecor_configuration;
 struct libdecor_interface;
 struct libdecor_frame_interface;
-enum libdecor_resize_edge;
-enum libdecor_capabilities;
-enum libdecor_window_state;
 
 #include <stdint.h>
 #include "wayland-cursor.h"