Fix compat with Truffleruby

Truffleruby doesn't support rb_check_symbol_cstr() so far, but all symbols in Truffleruby are GC'd anyway.
So we can use classic ID2SYM to allocate the Symbol objects.

Discussed here: oracle/truffleruby#1814

ext/pg_result.c

@@ -410,6 +410,7 @@ static VALUE pg_cstr_to_sym(char *cstr, unsigned int flags, int enc_idx)
 {
 	VALUE fname;
 	if( flags & PG_RESULT_FIELD_NAMES_SYMBOL ){
+#ifndef TRUFFLERUBY
 		rb_encoding *enc = rb_enc_from_index(enc_idx);
 		fname = rb_check_symbol_cstr(cstr, strlen(cstr), enc);
 		if( fname == Qnil ){
@@ -418,6 +419,7 @@ static VALUE pg_cstr_to_sym(char *cstr, unsigned int flags, int enc_idx)
 			fname = rb_str_intern(fname);
 		}
 	} else if( flags & PG_RESULT_FIELD_NAMES_STATIC_SYMBOL ){
+#endif
 		rb_encoding *enc = rb_enc_from_index(enc_idx);
 		fname = ID2SYM(rb_intern3(cstr, strlen(cstr), enc));
 	} else {