Add missing include guard

Extracted from bitcoin/bitcoin#12757
- Commit 8fd6af89a0c061d79bb3fd2ba704b61dcc2e9211

src/bech32.h

@@ -9,6 +9,9 @@
 //
 // For more information, see BIP 173.
 
+#ifndef BITCOIN_BECH32_H
+#define BITCOIN_BECH32_H
+
 #include <stdint.h>
 #include <string>
 #include <vector>
@@ -23,3 +26,5 @@ std::string Encode(const std::string& hrp, const std::vector<uint8_t>& values);
 std::pair<std::string, std::vector<uint8_t>> Decode(const std::string& str);
 
 } // namespace bech32
+
+#endif // BITCOIN_BECH32_H