[small fix] delete useless var in sm2_key.c

[KSroido]

to fix error

error: ../../common/gmssl/src/sm2_key.c: In function 'sm2_public_key_to_der': ../../common/gmssl/src/sm2_key.c:110:9: error: unused variable 'len' [-Werror=unused-variable]
  size_t len = 0;
         ^~~
cc1: all warnings being treated as errors

[KSroido]

FULL log:

"aarch64-poky-linux-gcc " -c -O3 -I../../include/L946 -I../../src/L946_cn/lib -I../../src/L946_cn/lib/mqtt/include -I../../src/L946_cn/daemon -I../../common/L946 -I../../third-party/L946_cn/ipc/include -I../../third-party/bidulibs/inc -I../../third-party/L946_cn/ipc/copy -I../../third-party/L946_cn/copy/include -I../../third-party/L946_cn/ipc/ipc/inc -I../../third-party/L946_cn/ipc/ipc/inc/autolib/inc -I../../common/gmssl/include -Wall -Werror -Waddress -frename-registers -ffunction-sections -fdata-sections -o ../.objs/idsApp/linux/aarch64/release/__/__/common/gmssl/src/hash_drbg.c.o ../../common/gmssl/src/hash_drbg.c
error: ../../common/gmssl/src/sm2_key.c: In function 'sm2_public_key_to_der':
../../common/gmssl/src/sm2_key.c:110:9: error: unused variable 'len' [-Werror=unused-variable]
  size_t len = 0;
         ^~~
cc1: all warnings being treated as errors
Error: xmake build failed!