Bind functions' availability for config options

Signed-off-by: Gabor Mezei <gabor.mezei@arm.com>
diff --git a/library/constant_time.h b/library/constant_time.h
index d2301aa..5796f12 100644
--- a/library/constant_time.h
+++ b/library/constant_time.h
@@ -92,6 +92,8 @@
 
 #endif /* MBEDTLS_BIGNUM_C */
 
+#if defined(MBEDTLS_SSL_SOME_SUITES_USE_TLS_CBC)
+
 /** Constant-flow mask generation for "greater or equal" comparison:
  * - if \p x >= \p y, return all-bits 1, that is (size_t) -1
  * - otherwise, return all bits 0, that is 0
@@ -108,6 +110,8 @@
 size_t mbedtls_cf_size_mask_ge( size_t x,
                                 size_t y );
 
+#endif /* MBEDTLS_SSL_SOME_SUITES_USE_TLS_CBC */
+
 /** Constant-flow boolean "equal" comparison:
  * return x == y
  *
@@ -122,7 +126,6 @@
 unsigned mbedtls_cf_size_bool_eq( size_t x,
                                   size_t y );
 
-
 #if defined(MBEDTLS_BIGNUM_C)
 
 /** Decide if an integer is less than the other, without branches.
@@ -176,6 +179,7 @@
 
 #endif /* MBEDTLS_BIGNUM_C */
 
+#if defined(MBEDTLS_SSL_SOME_SUITES_USE_TLS_CBC)
 
 /** Conditional memcpy without branches.
  *
@@ -219,8 +223,6 @@
                                size_t offset_max,
                                size_t len );
 
-#if defined(MBEDTLS_SSL_SOME_SUITES_USE_TLS_CBC)
-
 /** Compute the HMAC of variable-length data with constant flow.
  *
  * This function computes the HMAC of the concatenation of \p add_data and \p