Fix misc issues with unused parameters and check-names.sh

Fix unused parameter warnings when MBEDTLS_TEST_HOOKS is not enabled.

A few issues were caught by check-names.sh namely:

- mbedtls_error_add was not capitalised.
- mbedtls_test_hook_error_add was being defined multiple times as the
  definition was in a header.

Signed-off-by: Chris Jones <christopher.jones@arm.com>

library/error.c

@@ -893,6 +893,8 @@ const char * mbedtls_low_level_strerr( int error_code )
     return( NULL );
 }
 
+void (*mbedtls_test_hook_error_add)( int, int, const char *, int );
+
 void mbedtls_strerror( int ret, char *buf, size_t buflen )
 {
     size_t len;