Do not hide function prototype.

It is called from kernel crypt backend unconditionally with
the proper define in config.h but some static parrsers are not so clever.

Compilation will fail in linker phase anyway if wrongly used.

lib/crypto_backend/crypto_backend_internal.h

@@ -23,7 +23,6 @@
 
 #include "crypto_backend.h"
 
-#if USE_INTERNAL_PBKDF2
 /* internal PBKDF2 implementation */
 int pkcs5_pbkdf2(const char *hash,
 		 const char *P, size_t Plen,
@@ -31,7 +30,6 @@ int pkcs5_pbkdf2(const char *hash,
 		 unsigned int c,
 		 unsigned int dkLen, char *DK,
 		 unsigned int hash_block_size);
-#endif
 
 /* Argon2 implementation wrapper */
 int argon2(const char *type, const char *password, size_t password_length,