[backport#11140] Trivial: Improve #endif comments

Summary:
ac1cf8d Trivial: Improve #endif comments (danra)

Pull request description:

  Improve the #endif comments for the '#if HAVE_DECL_BSWAP_XX == 0' preprocessor conditions, so each shows the full condition which it closes.

---

Backport of Core [[bitcoin/bitcoin#11140 | PR11140]]

Test Plan:
  ninja

Reviewers: #bitcoin_abc, PiRK, Fabien

Reviewed By: #bitcoin_abc, PiRK, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8172

src/compat/byteswap.h

@@ -37,14 +37,14 @@
 inline uint16_t bswap_16(uint16_t x) {
     return (x >> 8) | (x << 8);
 }
-#endif // HAVE_DECL_BSWAP16
+#endif // HAVE_DECL_BSWAP16 == 0
 
 #if HAVE_DECL_BSWAP_32 == 0
 inline uint32_t bswap_32(uint32_t x) {
     return (((x & 0xff000000U) >> 24) | ((x & 0x00ff0000U) >> 8) |
             ((x & 0x0000ff00U) << 8) | ((x & 0x000000ffU) << 24));
 }
-#endif // HAVE_DECL_BSWAP32
+#endif // HAVE_DECL_BSWAP32 == 0
 
 #if HAVE_DECL_BSWAP_64 == 0
 inline uint64_t bswap_64(uint64_t x) {
@@ -57,7 +57,7 @@ inline uint64_t bswap_64(uint64_t x) {
             ((x & 0x000000000000ff00ull) << 40) |
             ((x & 0x00000000000000ffull) << 56));
 }
-#endif // HAVE_DECL_BSWAP64
+#endif // HAVE_DECL_BSWAP64 == 0
 
 #endif // defined(MAC_OSX)