mbedtls: Disable documentation warning as error with clang

There are shortcomings with doxygen info which clang-15+ flags, dont
treat them as errors

Remove unused variable

Fixes
library/bignum.c:1395:29: error: variable 't' set but not used [-Werror,-Wunused-but-set-variable]

Upstream-Status: Pending
Signed-off-by: Khem Raj <raj.khem@gmail.com>
--- a/third_party/openthread/repo/third_party/mbedtls/repo/library/bignum.c
+++ b/third_party/openthread/repo/third_party/mbedtls/repo/library/bignum.c
@@ -1392,7 +1392,7 @@ void mpi_mul_hlp( size_t i,
                   mbedtls_mpi_uint *d,
                   mbedtls_mpi_uint b )
 {
-    mbedtls_mpi_uint c = 0, t = 0;
+    mbedtls_mpi_uint c = 0, t __attribute__ ((unused)) = 0;
 
 #if defined(MULADDC_HUIT)
     for( ; i >= 8; i -= 8 )