crypto_backend.h: Wrap APIs in extern C

It's a good practice to mark public APIs as extern "C" so that
projects written in C++ can use our library.

[mbroz] It is not public API in cryptsetup, but we use this backend
in other projects, this aligns the code changes.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>

lib/crypto_backend/crypto_backend.h

@@ -21,6 +21,10 @@
 #define char16_t uint16_t
 #endif
 
+# ifdef __cplusplus
+extern "C" {
+# endif
+
 struct crypt_hash;
 struct crypt_hmac;
 struct crypt_cipher;
@@ -143,4 +147,8 @@ int crypt_backend_memeq(const void *m1, const void *m2, size_t n);
 /* crypto backend running in FIPS mode */
 bool crypt_fips_mode(void);
 
+# ifdef __cplusplus
+}
+# endif
+
 #endif /* _CRYPTO_BACKEND_H */