Fix implicit declaration of __builtin_bswap32

[ghost]

A friend without Gihub account gave me this:

diff —git a/mtproto-utils.c b/mtproto-utils.c
index 60572b3..ce6926e 100644
-— a/mtproto-utils.c
+++ b/mtproto-utils.c
@@ -3,6 +3,15 @@
 #include "tgl.h"
 #include "tools.h"

+#if !defined(HAVE___BUILTIN_BSWAP32) && !defined(__FreeBSD__) && !defined(__OpenBSD__)
+static inline unsigned __builtin_bswap32(unsigned x) {
+  return ((x « 24) & 0xff000000 ) |
+  ((x « 8) & 0x00ff0000 ) |
+  ((x » 8) & 0x0000ff00 ) |
+  ((x » 24) & 0x000000ff );
+}
+#endif
+
 static unsigned long long gcd (unsigned long long a, unsigned long long b) {
   return b ? gcd (b, a % b) : a;
 }

[BenWiederhake]

What problem does this solve? Is your friend aware that this seems to be a patch against the highly-outdated master branch? Could you tell your friend that the current test branch doesn't use that builtin anymore?