MINOR: lua: remove unused variable

MINOR: lua: remove unused variable

hlua_init() uses 'idx' only in openssl related code, while 'i' is used
in shared code and is safe to be reused. This commit replaces the use of
'idx' with 'i'

  $ make V=1 TARGET=linux-glibc USE_LUA=1 USE_OPENSSL=
  ..
  cc -Iinclude  -O2 -g -Wall -Wextra -Wdeclaration-after-statement -fwrapv -Wno-address-of-packed-member -Wno-unused-label -Wno-sign-compare -Wno-unused-parameter -Wno-clobbered -Wno-missing-field-initializers -Wno-cast-function-type  -Wtype-limits -Wshift-negative-value -Wshift-overflow=2 -Wduplicated-cond -Wnull-dereference   -DUSE_EPOLL  -DUSE_NETFILTER     -DUSE_POLL  -DUSE_THREAD  -DUSE_BACKTRACE   -DUSE_TPROXY -DUSE_LINUX_TPROXY -DUSE_LINUX_SPLICE -DUSE_LIBCRYPT -DUSE_CRYPT_H -DUSE_GETADDRINFO  -DUSE_LUA -DUSE_FUTEX -DUSE_ACCEPT4    -DUSE_CPU_AFFINITY -DUSE_TFO -DUSE_NS -DUSE_DL -DUSE_RT      -DUSE_PRCTL -DUSE_THREAD_DUMP    -I/usr/include/lua5.3 -I/usr/include/lua5.3  -DCONFIG_HAPROXY_VERSION=\"2.4-dev5-37286a-78\" -DCONFIG_HAPROXY_DATE=\"2021/01/21\" -c -o src/hlua.o src/hlua.c
  src/hlua.c: In function 'hlua_init':
  src/hlua.c:9145:6: warning: unused variable 'idx' [-Wunused-variable]
   9145 |  int idx;
        |      ^~~