compat/bswap: add include header guards

Our compat/bswap.h lacks the usual preprocessor guards against multiple
inclusion. This usually isn't an issue since it only gets included from
git-compat-util.h, which has its own guards. But it would produce
redeclaration errors if any file included it separately.

Our hdr-check target would complain about this, except that it currently
skips items in compat/ entirely.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

compat/bswap.h

+#ifndef COMPAT_BSWAP_H
+#define COMPAT_BSWAP_H
+
 /*
  * Let's make sure we always have a sane definition for ntohl()/htonl().
  * Some libraries define those as a function call, just to perform byte
@@ -210,3 +213,5 @@ static inline void put_be64(void *ptr, uint64_t value)
 }
 
 #endif
+
+#endif /* COMPAT_BSWAP_H */