Take Cryptographic API outside the XXX_ALT check

The cryptographic API should not be related to whether or not
there is alternative implementation. The API should be same for regular
implementation, and for alternative implementation, so it is defined
outside of the XXX_ALT precompilation check in the cryptographic API header
diff --git a/include/mbedtls/xtea.h b/include/mbedtls/xtea.h
index 34ccee3..8df708a 100644
--- a/include/mbedtls/xtea.h
+++ b/include/mbedtls/xtea.h
@@ -39,14 +39,14 @@
 #define MBEDTLS_ERR_XTEA_INVALID_INPUT_LENGTH             -0x0028  /**< The data input has an invalid length. */
 #define MBEDTLS_ERR_XTEA_HW_ACCEL_FAILED                  -0x0029  /**< XTEA hardware accelerator failed. */
 
-#if !defined(MBEDTLS_XTEA_ALT)
-// Regular implementation
-//
-
 #ifdef __cplusplus
 extern "C" {
 #endif
 
+#if !defined(MBEDTLS_XTEA_ALT)
+// Regular implementation
+//
+
 /**
  * \brief          XTEA context structure
  */
@@ -56,6 +56,10 @@
 }
 mbedtls_xtea_context;
 
+#else  /* MBEDTLS_XTEA_ALT */
+#include "xtea_alt.h"
+#endif /* MBEDTLS_XTEA_ALT */
+
 /**
  * \brief          Initialize XTEA context
  *
@@ -115,18 +119,6 @@
                     unsigned char *output);
 #endif /* MBEDTLS_CIPHER_MODE_CBC */
 
-#ifdef __cplusplus
-}
-#endif
-
-#else  /* MBEDTLS_XTEA_ALT */
-#include "xtea_alt.h"
-#endif /* MBEDTLS_XTEA_ALT */
-
-#ifdef __cplusplus
-extern "C" {
-#endif
-
 /**
  * \brief          Checkup routine
  *