Appease check-names with prefix

Adding an mbedtls_prefix is preferred to having '//no-check-names'
littered about.

Signed-off-by: David Horstmann <david.horstmann@arm.com>
diff --git a/library/cipher.c b/library/cipher.c
index 9be53b7..ebb03f4 100644
--- a/library/cipher.c
+++ b/library/cipher.c
@@ -839,9 +839,9 @@
     }
 }
 
-MBEDTLS_STATIC_TESTABLE int get_pkcs_padding(unsigned char *input,
-                                             size_t input_len,
-                                             size_t *data_len)
+MBEDTLS_STATIC_TESTABLE int mbedtls_get_pkcs_padding(unsigned char *input,
+                                                     size_t input_len,
+                                                     size_t *data_len)
 {
     size_t i, pad_idx;
     unsigned char padding_len;