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>

include/mbedtls/error.h

@@ -120,14 +120,14 @@ extern "C" {
  *        Wrapper function for mbedtls_err_add_ext(). See that function for
  *        more details.
  */
-#define mbedtls_error_add( high, low ) \
+#define MBEDTLS_ERROR_ADD( high, low ) \
         mbedtls_error_add_ext( high, low, __FILE__, __LINE__ )
 
 /**
  * \brief Testing hook called before adding/combining two error codes together.
  *        Only used when invasive testing is enabled via MBEDTLS_TEST_HOOKS.
  */
-void (*mbedtls_test_hook_error_add)( int, int, const char *, int );
+extern void (*mbedtls_test_hook_error_add)( int, int, const char *, int );
 
 /**
  * \brief Combines a high-level and low-level error code together.
@@ -150,6 +150,9 @@ static inline int mbedtls_error_add_ext( int high, int low,
     if( *mbedtls_test_hook_error_add != NULL )
         ( *mbedtls_test_hook_error_add )( high, low, file, line );
 #endif
+    (void)file;
+    (void)line;
+
     return( high + low );
 }