os: move C struct declarations in their own _default.c.v files (#12268)

vlib/os/os_structs_utsname_default.c.v

@@ -0,0 +1,15 @@
+module os
+
+struct C.utsname {
+mut:
+	sysname  &char
+	nodename &char
+	release  &char
+	version  &char
+	machine  &char
+}
+
+struct C.utimbuf {
+	actime  int
+	modtime int
+}