bignum_core: Extracted mbedtls_mpi_shift_l from prototype

Signed-off-by: Minos Galanakis <minos.galanakis@arm.com>
diff --git a/library/bignum_core.h b/library/bignum_core.h
index b3d05a3..25e7e1a 100644
--- a/library/bignum_core.h
+++ b/library/bignum_core.h
@@ -294,6 +294,18 @@
                               size_t count);
 
 /**
+ * \brief          Perform a left-shift on an MPI: X <<= count
+ *
+ * \param X        The MPI to shift. This must point to an initialized MPI.
+ * \param count    The number of bits to shift by.
+ *
+ * \return         \c 0 if successful.
+ * \return         #MBEDTLS_ERR_MPI_ALLOC_FAILED if a memory allocation failed.
+ * \return         Another negative error code on different kinds of failure.
+ */
+int mbedtls_mpi_shift_l( mbedtls_mpi *X, size_t count );
+
+/**
  * \brief Add two fixed-size large unsigned integers, returning the carry.
  *
  * Calculates `A + B` where `A` and `B` have the same size.