Move _exit function up, so gcc doesn't complain

litex/soc/software/libc/missing.c

@@ -4,15 +4,16 @@ int getentropy(void *v, size_t s) {
     return -1;
 }
 
-int getpid() {
+int getpid(void) {
     return 1;
 }
 
-int kill(int pid, int name) {
-    _exit(0);
-}
-
 void _exit(int code) {
     while (1);
 }
 
+int kill(int pid, int name) {
+    _exit(0);
+    return 0;
+}
+