crypto/bn: fix debug build

crypto/bn: fix debug build

Fixes:

    gcc  -I. -Iinclude -Iproviders/common/include -Iproviders/implementations/include -Icrypto/include  -DSTATIC_LEGACY -DBN_DEBUG -DREF_DEBUG     -DCONF_DEBUG -DBN_CTX_DEBUG -DOPENSSL_NO_ASM -ggdb -g2 -Wformat -Wshadow -Wmissing-prototypes -Wmissing-declarations -Werror      -DOPENSSL_BUILDING_OPENSSL -DOPENSSLDIR="\"/home/wsh/opt/openssl/ssl\"" -DENGINESDIR="\"/home/wsh/opt/openssl/lib/engines-3\""     -DMODULESDIR="\"/home/wsh/opt/openssl/lib/ossl-modules\""   -MMD -MF crypto/bn/libcrypto-lib-bn_conv.d.tmp -MT crypto/bn/libcrypto-lib-bn_conv.o -c -o crypto/bn/libcrypto-lib-bn_conv.o crypto/bn/bn_conv.c
    In file included from crypto/bn/bn_conv.c:12:
    crypto/bn/bn_conv.c: In function 'BN_hex2bn':
    crypto/bn/bn_local.h:178:31: error: implicit declaration of function 'ossl_assert'; did you mean 'ossl_ssize_t'? [-Werror=implicit-function-declaration]
                             (void)ossl_assert((_top == 0 && !_bnum2->neg) || \
                                   ^~~~~~~~~~~
    crypto/bn/bn_conv.c:188:5: note: in expansion of macro 'bn_check_top'
         bn_check_top(ret);
         ^~~~~~~~~~~~

CLA: trivial

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/10993)