hiredis_connection.c: Rename unix to is_unix

[jlledom]

The hiredis_reconnect function is defined like this:

static VALUE hiredis_reconnect(VALUE self, VALUE unix, VALUE ssl_param)

However, unix is defined as a macro on some implementations, I found the issue in RHEL7 for ppc64le architecture.

When the macro is defined, the preprocessor replaces it by a 1, and the function definition is transformed to this:

static VALUE hiredis_reconnect(VALUE self, VALUE 1, VALUE ssl_param) {

Which, on compile time fails with this error:

<command-line>:0:6: error: expected ';', ',' or ')' before numeric constant
hiredis_connection.c:595:50: note: in expansion of macro 'unix'
 static VALUE hiredis_reconnect(VALUE self, VALUE unix, VALUE ssl_param) {
                                                  ^

Just renaming the parameter fixes the issue for me.